diff --git a/src/anubis/parser/Parser.java b/src/anubis/parser/Parser.java
diff --git a/src/anubis/parser/ParserHelper.java b/src/anubis/parser/ParserHelper.java
@@ -14,7 +14,6 @@ public class ParserHelper {
 		"record",
 		"function",
 		"end",
-		"def",
 		"if",
 		"then",
 		"else",

diff --git a/src/anubis/parser/Scanner.java b/src/anubis/parser/Scanner.java
diff --git a/src/anubis/parser/anubis.jay b/src/anubis/parser/anubis.jay
@@ -19,7 +19,7 @@ public class Parser {
 // 構造キーワード
 %token<Token>		OBJECT, RECORD, FUNCTION
 // 制御構造キーワード
-%token<Token>		DEF, IF, THEN, ELSE, FOR, WHILE, DO, TRY, CATCH, FINALLY, SWITCH, CASE, ASSERT, USING, LOCK, END
+%token<Token>		IF, THEN, ELSE, FOR, WHILE, DO, TRY, CATCH, FINALLY, SWITCH, CASE, ASSERT, USING, LOCK, END
 // 制御構文キーワード
 %token<Token>		BREAK, CONTINUE, THROW, RETURN, ATMARK
 // 演算子
@@ -93,8 +93,6 @@ stmt:
 		{ $$ = new ForStatement($4, $6, $1, EmptyStatement.INSTANCE).setPos($3.getPos()); }
 	| cmdStmt ATMARK WHILE expr SEMICOLON
 		{ $$ = new WhileStatement($4, $1, EmptyStatement.INSTANCE).setPos($3.getPos()); }
-	| DEF leftvalue ASSIGN expr SEMICOLON
-		{ $$ = new ExpressionStatement(newNewSlotCall($2, $4, true)).setPos($1.getPos()); }
 	| labelOpt IF expr THEN stmtList elseStmtOpt END
 		{ $$ = new IfStatement($1, $3, $5, $6).setPos($2.getPos()); }
 	| labelOpt FOR id COLON expr DO stmtListOpt elseStmtOpt END

diff --git a/src/anubis/parser/anubis.jflex b/src/anubis/parser/anubis.jflex
@@ -96,7 +96,6 @@ WSP			= [\ \t]
 	"record"	{ return tk(Parser.RECORD); }
 	"function"	{ return tk(Parser.FUNCTION); }
 	"end"		{ return tk(Parser.END); }
-	"def"		{ return tk(Parser.DEF); }
 	"if"		{ return tk(Parser.IF); }
 	"then"		{ return tk(Parser.THEN); }
 	"else"		{ return tk(Parser.ELSE); }

diff --git a/src/anubis/runtime/ABuiltinFunction.java b/src/anubis/runtime/ABuiltinFunction.java
@@ -27,7 +27,7 @@ public String debugString() {
 	}
 
 	public ABuiltinFunction register(AnubisObject owner, String name) {
-		owner.setSlot(name, this, true);
+		owner.setSlot(name, this);
 		return this;
 	}
 

diff --git a/src/anubis/runtime/ANop.java b/src/anubis/runtime/ANop.java
@@ -118,11 +118,6 @@ public void setSlot(String name, AnubisObject value) {
 		;
 	}
 
-	@Override
-	public void setSlot(String name, AnubisObject value, boolean readonly) {
-		;
-	}
-
 	@Override
 	public String toString() {
 		return debugString();

diff --git a/src/anubis/runtime/ARecord.java b/src/anubis/runtime/ARecord.java
@@ -20,19 +20,12 @@ public AnubisObject get() {
 		public void set(AnubisObject value) {
 			; // nop
 		}
-
-		@Override
-		public void set(AnubisObject value, boolean readonly) {
-			; // nop
-		}
 	}
 
 	public abstract class Field {
 		public abstract AnubisObject get();
 
 		public abstract void set(AnubisObject value);
-
-		public abstract void set(AnubisObject value, boolean readonly);
 	}
 
 	public class NestedField extends Field {
@@ -48,13 +41,6 @@ public void set(AnubisObject value) {
 			// TODO Auto-generated method stub
 
 		}
-
-		@Override
-		public void set(AnubisObject value, boolean readonly) {
-			// TODO Auto-generated method stub
-
-		}
-
 	}
 
 	public class VariableField extends Field {
@@ -74,10 +60,5 @@ public AnubisObject get() {
 		public void set(AnubisObject value) {
 			ARecord.this.setSlot(name, value);
 		}
-
-		@Override
-		public void set(AnubisObject value, boolean readonly) {
-			ARecord.this.setSlot(name, value, readonly);
-		}
 	}
 }
diff --git a/src/anubis/runtime/AbstractAObject.java b/src/anubis/runtime/AbstractAObject.java
@@ -4,7 +4,7 @@
 import anubis.SlotRef;
 import anubis.SpecialSlot;
 import anubis.except.ExceptionProvider;
-import anubis.except.SlotReadonlyException;
+import anubis.except.ObjectFreezeException;
 
 public abstract class AbstractAObject implements AnubisObject {
 	protected final SlotTable slots;
@@ -78,7 +78,7 @@ public void setSlot(SpecialSlot type, AnubisObject value) {
 				this._outer = value;
 				break;
 			case THIS:
-				throw ExceptionProvider.newSlotReadonly(this, "this");
+				throw ExceptionProvider.newObjectFreeze(this, "this");
 			default:
 				throw ExceptionProvider.newSlotNotFound(this, type.toString());
 		}
@@ -87,21 +87,10 @@ public void setSlot(SpecialSlot type, AnubisObject value) {
 	@Override
 	public void setSlot(String name, AnubisObject value) {
 		try {
-			slots.put(name, value, false);
+			slots.put(name, value);
 		}
-		catch (SlotReadonlyException ex) {
-			throw ExceptionProvider.newSlotReadonly(ex, this, name);
+		catch (ObjectFreezeException ex) {
+			throw ExceptionProvider.newObjectFreeze(ex, this, name);
 		}
 	}
-
-	@Override
-	public void setSlot(String name, AnubisObject value, boolean readonly) {
-		try {
-			slots.put(name, value, readonly);
-		}
-		catch (SlotReadonlyException ex) {
-			throw ExceptionProvider.newSlotReadonly(ex, this, name);
-		}
-	}
-
 }
diff --git a/src/anubis/runtime/AbstractSlotTable.java b/src/anubis/runtime/AbstractSlotTable.java
@@ -1,61 +1,20 @@
 package anubis.runtime;
 
-import java.util.Collections;
-import java.util.HashSet;
-import java.util.Map;
-import java.util.Set;
-import anubis.AnubisObject;
 import anubis.except.ExceptionProvider;
 
 /**
  * @author SiroKuro
  */
 public abstract class AbstractSlotTable implements SlotTable {
-	private Set<String> readonly = null;
 	private volatile boolean immutable = false;
 
 	public void freeze() {
 		this.immutable = true;
 	}
 
-	@Override
-	public synchronized SnapShot getSnap() {
-		final Map<String, AnubisObject> slots = getSlotSnaps();
-		final Set<String> readonlys = getReadonlySnaps();
-		return new SnapShot() {
-			@Override
-			public Set<String> getReadonlySlots() {
-				return readonlys;
-			}
-
-			@Override
-			public Map<String, AnubisObject> getSlots() {
-				return slots;
-			}
-		};
-	}
-
-	protected synchronized void assertNotReadonly(String name, boolean setReadonly) {
-		if (immutable || (readonly != null && readonly.contains(name))) {
-			throw ExceptionProvider.newSlotReadonly(name);
-		}
-		if (setReadonly) {
-			prepareReadonlySet().add(name);
-		}
-	}
-
-	protected Set<String> getReadonlySnaps() {
-		if (readonly == null)
-			return Collections.emptySet();
-		return Collections.unmodifiableSet(new HashSet<String>(readonly));
-	}
-
-	protected abstract Map<String, AnubisObject> getSlotSnaps();
-
-	private Set<String> prepareReadonlySet() {
-		if (this.readonly == null) {
-			this.readonly = new HashSet<String>();
+	protected synchronized void assertNotFreeze() {
+		if (immutable) {
+			throw ExceptionProvider.newObjectFreeze();
 		}
-		return this.readonly;
 	}
 }
diff --git a/src/anubis/runtime/JClass.java b/src/anubis/runtime/JClass.java
@@ -68,7 +68,7 @@ protected static SlotTable newClassTable(Class<?> cls) {
 		// traits へ変換
 		SlotTable result = new SimpleSlotTable();
 		for (Entry<String, FunctionAccessor> ent: accessors.entrySet()) {
-			result.put(ent.getKey(), AObjects.attachTraits(JFunction.valueOf(ent.getValue())), true);
+			result.put(ent.getKey(), AObjects.attachTraits(JFunction.valueOf(ent.getValue())));
 		}
 		return result;
 	}

diff --git a/src/anubis/runtime/SimpleSlotTable.java b/src/anubis/runtime/SimpleSlotTable.java
@@ -11,28 +11,30 @@
 public class SimpleSlotTable extends AbstractSlotTable {
 	private volatile Map<String, AnubisObject> internal = null;
 
+	@Override
 	public AnubisObject get(String name) {
 		if (internal != null) {
 			return internal.get(name);
 		}
 		return null;
 	}
 
-	public synchronized void put(String name, AnubisObject value, boolean setReadonly) {
-		assertNotReadonly(name, setReadonly);
-		if (value == null)
-			prepareInternal().remove(name);
-		else
-			prepareInternal().put(name, value);
-	}
-
 	@Override
-	protected Map<String, AnubisObject> getSlotSnaps() {
+	public Map<String, AnubisObject> getSnap() {
 		if (internal == null)
 			return Collections.emptyMap();
 		return Collections.unmodifiableMap(new HashMap<String, AnubisObject>(internal));
 	}
 
+	@Override
+	public synchronized void put(String name, AnubisObject value) {
+		assertNotFreeze();
+		if (value == null)
+			prepareInternal().remove(name);
+		else
+			prepareInternal().put(name, value);
+	}
+
 	private Map<String, AnubisObject> prepareInternal() {
 		if (this.internal == null) {
 			this.internal = new HashMap<String, AnubisObject>();

diff --git a/src/anubis/runtime/SlotTable.java b/src/anubis/runtime/SlotTable.java
@@ -1,7 +1,6 @@
 package anubis.runtime;
 
 import java.util.Map;
-import java.util.Set;
 import anubis.AnubisObject;
 
 /**
@@ -13,13 +12,7 @@ public interface SlotTable {
 
 	public AnubisObject get(String name);
 
-	public SnapShot getSnap();
+	public Map<String, AnubisObject> getSnap();
 
-	public void put(String name, AnubisObject value, boolean setReadonly);
-
-	public interface SnapShot {
-		public Set<String> getReadonlySlots();
-
-		public Map<String, AnubisObject> getSlots();
-	}
+	public void put(String name, AnubisObject value);
 }
diff --git a/src/anubis/runtime/Utils.java b/src/anubis/runtime/Utils.java
@@ -4,14 +4,14 @@
 import java.io.StringWriter;
 import java.math.BigDecimal;
 import java.math.BigInteger;
+import java.util.Map;
 import java.util.Map.Entry;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicLong;
 import anubis.AnubisObject;
 import anubis.SpecialSlot;
 import anubis.except.ExceptionProvider;
 import anubis.parser.ParserHelper;
-import anubis.runtime.SlotTable.SnapShot;
 
 /**
  * @author SiroKuro
@@ -60,9 +60,9 @@ public static int asIntArgs(AnubisObject obj, int _default) {
 		return num.getNumber().intValue();
 	}
 
-	public static String formatDumpString(AObject object, SnapShot snapShot) {
+	public static String formatDumpString(AObject object, Map<String, AnubisObject> snaps) {
 		int max_length = 5;
-		for (String key: snapShot.getSlots().keySet()) {
+		for (String key: snaps.keySet()) {
 			key = ParserHelper.quoteIdentifier(key);
 			if (max_length < key.length()) {
 				max_length = key.length();
@@ -73,7 +73,7 @@ public static String formatDumpString(AObject object, SnapShot snapShot) {
 		StringWriter sw = new StringWriter();
 		PrintWriter pw = new PrintWriter(sw);
 		{
-			pw.println(object.debugString());
+			pw.println(object.debugString() + " {");
 			// 特殊スロットのダンプ
 			AnubisObject _this = object.getSlot(SpecialSlot.THIS);
 			AnubisObject _super = object.getSlot(SpecialSlot.SUPER);
@@ -85,15 +85,9 @@ public static String formatDumpString(AObject object, SnapShot snapShot) {
 			if (_outer != null)
 				pw.println(padding(max_length, "    outer") + " = " + Operator.toDebugString(_outer));
 			// 一般スロットのダンプ
-			for (Entry<String, AnubisObject> elm: snapShot.getSlots().entrySet()) {
-				if (snapShot.getReadonlySlots().contains(elm.getKey())) {
-					pw.println(padding(max_length, "R   " + ParserHelper.quoteIdentifier(elm.getKey())) + " = "
-							+ Operator.toDebugString(elm.getValue()));
-				}
-				else {
-					pw.println(padding(max_length, "    " + ParserHelper.quoteIdentifier(elm.getKey())) + " = "
-							+ Operator.toDebugString(elm.getValue()));
-				}
+			for (Entry<String, AnubisObject> elm: snaps.entrySet()) {
+				pw.println(padding(max_length, "    " + ParserHelper.quoteIdentifier(elm.getKey())) + " = "
+						+ Operator.toDebugString(elm.getValue()));
 			}
 			pw.println("}");
 		}

diff --git a/src/anubis/runtime/builtin/ARootNewSlot.java b/src/anubis/runtime/builtin/ARootNewSlot.java
@@ -1,20 +1,19 @@
 package anubis.runtime.builtin;
 
 import anubis.AnubisObject;
-import anubis.runtime.ABuiltinFunction._3;
+import anubis.runtime.ABuiltinFunction._2;
 import anubis.runtime.AString;
-import anubis.runtime.Operator;
 import anubis.runtime.Utils;
 
-public class ARootNewSlot extends _3 {
+public class ARootNewSlot extends _2 {
 	public ARootNewSlot(AnubisObject owner, String name) {
 		super(owner, name);
 	}
 
 	@Override
-	protected AnubisObject exec(AnubisObject _this, AnubisObject name, AnubisObject value, AnubisObject readonly) {
+	protected AnubisObject exec(AnubisObject _this, AnubisObject name, AnubisObject value) {
 		AString text = Utils.as(AString.class, name);
-		_this.setSlot(text.getValue(), value, Operator.isTrue(readonly));
+		_this.setSlot(text.getValue(), value);
 		return value;
 	}
 }
diff --git a/src/anubis/runtime/java/BindingsSlotTable.java b/src/anubis/runtime/java/BindingsSlotTable.java
@@ -31,18 +31,7 @@ public AnubisObject get(String key) {
 	}
 
 	@Override
-	public void put(String name, AnubisObject value, boolean setReadonly) {
-		synchronized (bindings) {
-			assertNotReadonly(name, setReadonly);
-			if (value == null)
-				bindings.remove(name);
-			else
-				bindings.put(name, JCaster.cast(value));
-		}
-	}
-
-	@Override
-	protected Map<String, AnubisObject> getSlotSnaps() {
+	public Map<String, AnubisObject> getSnap() {
 		synchronized (bindings) {
 			Map<String, AnubisObject> result = new TreeMap<String, AnubisObject>();
 			for (Entry<String, Object> elm: bindings.entrySet()) {
@@ -51,4 +40,15 @@ protected Map<String, AnubisObject> getSlotSnaps() {
 			return Collections.unmodifiableMap(result);
 		}
 	}
+
+	@Override
+	public void put(String name, AnubisObject value) {
+		synchronized (bindings) {
+			assertNotFreeze();
+			if (value == null)
+				bindings.remove(name);
+			else
+				bindings.put(name, JCaster.cast(value));
+		}
+	}
 }
diff --git a/src/anubis/runtime/java/JFieldSlotTable.java b/src/anubis/runtime/java/JFieldSlotTable.java
@@ -56,33 +56,33 @@ public AnubisObject get(String name) {
 	}
 
 	@Override
-	public void put(String name, AnubisObject value, boolean setReadonly) {
-		assertNotReadonly(name, setReadonly);
-		Field ff = fields.get(name);
-		if (ff != null && Modifier.isFinal(ff.getModifiers()) == false) {
+	public Map<String, AnubisObject> getSnap() {
+		Map<String, AnubisObject> result = new TreeMap<String, AnubisObject>();
+		for (Field ff: fields.values()) {
 			try {
-				ff.set(object, JCaster.cast(ff.getType(), value));
+				result.put(ff.getName(), AObjects.getObject(ff.get(object)));
 			}
 			catch (IllegalAccessException ex) {
 				throw ExceptionProvider.wrapRuntimeException(ex);
 			}
 		}
-		else {
-			throw ExceptionProvider.newSlotReadonly(name);
-		}
+		return Collections.unmodifiableMap(result);
 	}
 
 	@Override
-	protected Map<String, AnubisObject> getSlotSnaps() {
-		Map<String, AnubisObject> result = new TreeMap<String, AnubisObject>();
-		for (Field ff: fields.values()) {
+	public void put(String name, AnubisObject value) {
+		assertNotFreeze();
+		Field ff = fields.get(name);
+		if (ff != null && Modifier.isFinal(ff.getModifiers()) == false) {
 			try {
-				result.put(ff.getName(), AObjects.getObject(ff.get(object)));
+				ff.set(object, JCaster.cast(ff.getType(), value));
 			}
 			catch (IllegalAccessException ex) {
 				throw ExceptionProvider.wrapRuntimeException(ex);
 			}
 		}
-		return Collections.unmodifiableMap(result);
+		else {
+			throw ExceptionProvider.newObjectFreeze(name);
+		}
 	}
 }
diff --git a/src/anubis/runtime/java/ScriptContextSlotTable.java b/src/anubis/runtime/java/ScriptContextSlotTable.java
@@ -31,18 +31,7 @@ public AnubisObject get(String name) {
 	}
 
 	@Override
-	public void put(String name, AnubisObject value, boolean setReadonly) {
-		synchronized (context) {
-			assertNotReadonly(name, setReadonly);
-			if (value == null)
-				context.removeAttribute(name, ScriptContext.ENGINE_SCOPE);
-			else
-				context.setAttribute(name, JCaster.cast(value), ScriptContext.ENGINE_SCOPE);
-		}
-	}
-
-	@Override
-	protected Map<String, AnubisObject> getSlotSnaps() {
+	public Map<String, AnubisObject> getSnap() {
 		synchronized (context) {
 			Map<String, AnubisObject> result = new TreeMap<String, AnubisObject>();
 			List<Integer> scopes = new ArrayList<Integer>(context.getScopes());
@@ -56,4 +45,15 @@ protected Map<String, AnubisObject> getSlotSnaps() {
 			return Collections.unmodifiableMap(result);
 		}
 	}
+
+	@Override
+	public void put(String name, AnubisObject value) {
+		synchronized (context) {
+			assertNotFreeze();
+			if (value == null)
+				context.removeAttribute(name, ScriptContext.ENGINE_SCOPE);
+			else
+				context.setAttribute(name, JCaster.cast(value), ScriptContext.ENGINE_SCOPE);
+		}
+	}
 }
diff --git a/test/anubis/AnubisEngineTest.java b/test/anubis/AnubisEngineTest.java
@@ -28,8 +28,6 @@ public void testEval() throws Exception {
 		assertEquals("abc", engine.eval("x"));
 		assertEquals("abcdef", engine.eval("x += 'def'"));
 		assertEquals("abcdef", engine.eval("x"));
-		assertEquals("ABC", engine.eval("def text = 'ABC'; return text;"));
-		assertEquals("ABC", engine.eval("text"));
 	}
 
 	/**

diff --git a/test/anubis/except/ExceptionProviderTest.java b/test/anubis/except/ExceptionProviderTest.java
@@ -109,32 +109,32 @@ public void testNewSlotNotFound() {
 	}
 
 	/**
-	 * {@link anubis.except.ExceptionProvider#newSlotReadonly(java.lang.String)} のためのテスト・メソッド。
+	 * {@link anubis.except.ExceptionProvider#newObjectFreeze(java.lang.String)} のためのテスト・メソッド。
 	 */
 	@Test
 	public void testNewSlotReadonly1() {
-		SlotReadonlyException result = ExceptionProvider.newSlotReadonly("abc");
+		ObjectFreezeException result = ExceptionProvider.newObjectFreeze("abc");
 		assertNotNull(result);
 		System.out.println(result.getMessage());
 	}
 
 	/**
-	 * {@link anubis.except.ExceptionProvider#newSlotReadonly(anubis.AnubisObject, java.lang.String)} のためのテスト・メソッド。
+	 * {@link anubis.except.ExceptionProvider#newObjectFreeze(anubis.AnubisObject, java.lang.String)} のためのテスト・メソッド。
 	 */
 	@Test
 	public void testNewSlotReadonly2() {
-		SlotReadonlyException result = ExceptionProvider.newSlotReadonly(object, "abc");
+		ObjectFreezeException result = ExceptionProvider.newObjectFreeze(object, "abc");
 		assertNotNull(result);
 		System.out.println(result.getMessage());
 	}
 
 	/**
-	 * {@link anubis.except.ExceptionProvider#newSlotReadonly(java.lang.Throwable, anubis.AnubisObject, java.lang.String)} のためのテスト・メソッド。
+	 * {@link anubis.except.ExceptionProvider#newObjectFreeze(java.lang.Throwable, anubis.AnubisObject, java.lang.String)} のためのテスト・メソッド。
 	 */
 	@Test
 	public void testNewSlotReadonly3() {
 		Exception cause = new Exception("sample exception");
-		SlotReadonlyException result = ExceptionProvider.newSlotReadonly(cause, object, "abc");
+		ObjectFreezeException result = ExceptionProvider.newObjectFreeze(cause, object, "abc");
 		assertNotNull(result);
 		assertEquals(cause, result.getCause());
 		System.out.println(result.getMessage());

diff --git a/test/anubis/parser/ParserTest.java b/test/anubis/parser/ParserTest.java
@@ -130,7 +130,6 @@ public void testParseStatement() throws Exception {
 		parse("continue;");
 		parse("continue LABEL;");
 		parse("throw null;");
-		parse("def x = 123;");
 		parse("assert x == 0;");
 		parse("x = 1 + 2 + 3 @if x == 1;");
 		parse("return 123 @if x == 1;");

diff --git a/test/anubis/runtime/AObjectTest.java b/test/anubis/runtime/AObjectTest.java
@@ -6,7 +6,6 @@
 import static org.junit.Assert.assertSame;
 import org.junit.Test;
 import anubis.AnubisObject;
-import anubis.except.SlotReadonlyException;
 
 /**
  * @author SiroKuro
@@ -205,23 +204,4 @@ public void testGetSpecialSlot() {
 		assertNull(obj.getSlot(SUPER));
 		assertNull(obj.getSlot(OUTER));
 	}
-
-	/**
-	 * 読み取り専用スロットの確認
-	 */
-	@Test
-	public void testSetSlotReadonly() {
-		obj.setSlot("a", obj2, true);
-		assertSame(obj2, obj.getSlot("a"));
-	}
-
-	/**
-	 * 読み取り専用スロットに再設定した時の確認
-	 */
-	@Test(expected = SlotReadonlyException.class)
-	public void testSetSlotReadonly02() {
-		obj.setSlot("a", obj2, true);
-		assertSame(obj2, obj.getSlot("a"));
-		obj.setSlot("a", obj3);
-	}
 }
diff --git a/test/anubis/runtime/JObjectTest.java b/test/anubis/runtime/JObjectTest.java
@@ -6,7 +6,7 @@
 import anubis.AbstractTest;
 import anubis.AnubisObject;
 import anubis.SpecialSlot;
-import anubis.except.SlotReadonlyException;
+import anubis.except.ObjectFreezeException;
 import anubis.runtime.java.JCaster;
 
 /**
@@ -27,7 +27,7 @@ public void testFieldAccess01() throws Exception {
 		assertAEquals(4, exec("obj.getY()"));
 	}
 
-	@Test(expected = SlotReadonlyException.class)
+	@Test(expected = ObjectFreezeException.class)
 	public void testFieldAccess02() throws Exception {
 		local.setSlot("cls", AObjects.getObject(Sample.class));