diff --git a/evaluator/evaluator_test.go b/evaluator/evaluator_test.go
index 7c4c01c..aa555e9 100644
--- a/evaluator/evaluator_test.go
+++ b/evaluator/evaluator_test.go
@@ -631,10 +631,10 @@ func TestMathModule(t *testing.T) {
 		input    string
 		expected interface{}
 	}{
-		{`Math.abs(123)`, 123},
-		{`Math.abs(-123)`, 123},
-		{`Math.abs("foo")`, "first argument to 'Math.abs' must be NUMBER, got STRING on line 1"},
-		{`Math.abs()`, "wrong number of arguments: 0 while expected: 1 on line 1"},
+		{`math.abs(123)`, 123},
+		{`math.abs(-123)`, 123},
+		{`math.abs("foo")`, "first argument to 'Math.abs' must be NUMBER, got STRING on line 1"},
+		{`math.abs()`, "wrong number of arguments: 0 while expected: 1 on line 1"},
 	}
 
 	for _, tt := range tests {
diff --git a/lexer/lexer.go b/lexer/lexer.go
index ed60527..60630cb 100644
--- a/lexer/lexer.go
+++ b/lexer/lexer.go
@@ -293,6 +293,18 @@ func (lexer *Lexer) readNumber() string {
 		lexer.readCharacter()
 	}
 
+	if lexer.character == rune('e') {
+		lexer.readCharacter()
+
+		if lexer.character == rune('-') {
+			lexer.readCharacter()
+		}
+
+		for isDigit(lexer.character) {
+			lexer.readCharacter()
+		}
+	}
+
 	return string(lexer.input[position:lexer.position])
 }
 
diff --git a/lexer/lexer_test.go b/lexer/lexer_test.go
index 25a5058..eeef125 100644
--- a/lexer/lexer_test.go
+++ b/lexer/lexer_test.go
@@ -58,6 +58,8 @@ for (key, value in data) {
 }
 こんにちは
 世界
+123e4
+123e-4
 `
 
 	tests := []struct {
@@ -200,6 +202,8 @@ for (key, value in data) {
 		{token.RBRACE, "}", 49},
 		{token.IDENTIFIER, "こんにちは", 50},
 		{token.IDENTIFIER, "世界", 51},
+		{token.NUMBER, "123e4", 52},
+		{token.NUMBER, "123e-4", 53},
 		{token.EOF, "", 0},
 	}