diff --git a/src/main/java/org/komamitsu/fluency/buffer/PackedForwardBuffer.java b/src/main/java/org/komamitsu/fluency/buffer/PackedForwardBuffer.java
index 48462e51..68018b0b 100644
--- a/src/main/java/org/komamitsu/fluency/buffer/PackedForwardBuffer.java
+++ b/src/main/java/org/komamitsu/fluency/buffer/PackedForwardBuffer.java
@@ -50,15 +50,17 @@ private RetentionBuffer prepareBuffer(String tag, int writeSize)
             return retentionBuffer;
         }
 
+        int existingDataSize = 0;
         int newRetentionBufferSize;
         if (retentionBuffer == null) {
             newRetentionBufferSize = bufferConfig.getInitialBufferSize();
         }
         else{
+            existingDataSize = retentionBuffer.getByteBuffer().position();
             newRetentionBufferSize = (int) (retentionBuffer.getByteBuffer().capacity() * bufferConfig.getBufferExpandRatio());
         }
 
-        while (newRetentionBufferSize < writeSize) {
+        while (newRetentionBufferSize < (writeSize + existingDataSize)) {
             newRetentionBufferSize *= bufferConfig.getBufferExpandRatio();
         }
 
diff --git a/src/test/java/org/komamitsu/fluency/buffer/PackedForwardBufferTest.java b/src/test/java/org/komamitsu/fluency/buffer/PackedForwardBufferTest.java
index 9dfd2963..619912a7 100644
--- a/src/test/java/org/komamitsu/fluency/buffer/PackedForwardBufferTest.java
+++ b/src/test/java/org/komamitsu/fluency/buffer/PackedForwardBufferTest.java
@@ -66,4 +66,35 @@ public void testGetBufferedDataSize()
         buffer.flush(sender, true);
         assertThat(buffer.getBufferedDataSize(), is(0L));
     }
+
+    @Test
+    public void testAppendIfItDoesNotThrowBufferOverflow()
+            throws IOException
+    {
+        PackedForwardBuffer buffer = new PackedForwardBuffer.Config().setInitialBufferSize(64 * 1024).createInstance();
+
+        StringBuilder buf = new StringBuilder();
+
+        for (int i = 0; i < 1024 * 60; i++) {
+            buf.append('x');
+        }
+        String str60kb = buf.toString();
+
+        for (int i = 0; i < 1024 * 40; i++) {
+            buf.append('x');
+        }
+        String str100kb = buf.toString();
+
+        {
+            Map<String, Object> map = new HashMap<String, Object>();
+            map.put("k", str60kb);
+            buffer.append("tag0", new Date().getTime(), map);
+        }
+
+        {
+            Map<String, Object> map = new HashMap<String, Object>();
+            map.put("k", str100kb);
+            buffer.append("tag0", new Date().getTime(), map);
+        }
+    }
 }
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