diff --git a/ChangeLog b/ChangeLog
index 2d85de32e1..ab223a1501 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Mon Apr 13 11:35:55 2009  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* sprintf.c (rb_str_format): optimize previous commit.
+	  [ruby-list:45954]
+
 Mon Apr 13 10:58:54 2009  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* sprintf.c (rb_str_format): check encoding compatibility only on
diff --git a/sprintf.c b/sprintf.c
index 2bcf138c27..c276761bf2 100644
--- a/sprintf.c
+++ b/sprintf.c
@@ -660,7 +660,8 @@ rb_str_format(int argc, const VALUE *argv, VALUE fmt)
 		str = rb_obj_as_string(arg);
 		if (OBJ_TAINTED(str)) tainted = 1;
 		len = RSTRING_LEN(str);
-		enc = rb_enc_check(rb_str_substr(result, 0, blen), str);
+		rb_str_set_len(result, blen);
+		enc = rb_enc_check(result, str);
 		if (flags&(FPREC|FWIDTH)) {
 		    slen = rb_enc_strlen(RSTRING_PTR(str),RSTRING_END(str),enc);
 		    if (slen < 0) {