diff --git a/ChangeLog b/ChangeLog
index 67b8869151..898e7191f9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Aug  1 19:23:27 2009  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* string.c (tr_trans): change condition of singlebyte
+	  optimization.
+
 Sat Aug  1 18:50:53 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* random.c (random_float): rejects Infinity and NaN.
diff --git a/string.c b/string.c
index 6e6db0053a..06edf3b8d1 100644
--- a/string.c
+++ b/string.c
@@ -4871,8 +4871,7 @@ tr_trans(VALUE str, VALUE src, VALUE repl, int sflag)
 	    if (r == errc) r = trrepl.now;
 	    if (c < 256) {
 		trans[c] = r;
-		if (r > 255 || (r > 127 && rb_enc_mbmaxlen(enc) != 1))
-		    singlebyte = 0;
+		if (rb_enc_codelen(r, enc) != 1) singlebyte = 0;
 	    }
 	    else {
 		if (!hash) hash = rb_hash_new();