diff --git a/ChangeLog b/ChangeLog
index e5ccda7bb7..7445e5f6d0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Sep 21 00:47:44 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* string.c (rb_str_conv_enc_opts): make sure to scan coderange to get
+	  rid of unnecessary conversion.
+
 Sat Sep 21 00:21:08 2013  Zachary Scott  <e@zzak.io>
 
 	* ext/openssl/lib/openssl/ssl.rb: [DOC] Document OpenSSL::SSLServer
diff --git a/string.c b/string.c
index 1fd057151b..1b64425977 100644
--- a/string.c
+++ b/string.c
@@ -554,7 +554,7 @@ rb_str_conv_enc_opts(VALUE str, rb_encoding *from, rb_encoding *to, int ecflags,
     if (!to) return str;
     if (!from) from = rb_enc_get(str);
     if (from == to) return str;
-    if ((rb_enc_asciicompat(to) && ENC_CODERANGE(str) == ENC_CODERANGE_7BIT) ||
+    if ((rb_enc_asciicompat(to) && is_ascii_string(str)) ||
 	to == rb_ascii8bit_encoding()) {
 	if (STR_ENC_GET(str) != to) {
 	    str = rb_str_dup(str);