diff --git a/ChangeLog b/ChangeLog
index 1230329aff..57b405dee7 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Mon Sep 15 02:37:19 2008  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* transcode.c (str_encode): returns duplicated string if nothing
+	  changed.  [ruby-core:18578]
+
 Sun Sep  14 22:09:01 2008  Takeyuki Fujioka  <xibbar@ruby-lang.org>
 
 	* lib/cgi/core.rb (CGI::parse): performance improvement
diff --git a/test/ruby/test_transcode.rb b/test/ruby/test_transcode.rb
index 5abeed88ee..8770328bc1 100644
--- a/test/ruby/test_transcode.rb
+++ b/test/ruby/test_transcode.rb
@@ -582,4 +582,10 @@ class TestTranscode < Test::Unit::TestCase
     assert_raise(Encoding::ConversionUndefined) { "\u203E".encode("CP51932") }
   end
 
+  def test_nothing_changed
+    a = "James".force_encoding("US-ASCII")
+    b = a.encode("Shift_JIS")
+    assert_equal(Encoding::US_ASCII, a.encoding)
+    assert_equal(Encoding::Shift_JIS, b.encoding)
+  end
 end
diff --git a/transcode.c b/transcode.c
index 4d1107ddc4..1c1126fcce 100644
--- a/transcode.c
+++ b/transcode.c
@@ -2570,7 +2570,12 @@ str_encode(int argc, VALUE *argv, VALUE str)
     int encidx = str_transcode(argc, argv, &newstr);
 
     if (encidx < 0) return rb_str_dup(str);
-    RBASIC(newstr)->klass = rb_obj_class(str);
+    if (newstr == str) {
+	newstr = rb_str_dup(str);
+    }
+    else {
+	RBASIC(newstr)->klass = rb_obj_class(str);
+    }
     return str_encode_associate(newstr, encidx);
 }