diff --git a/ChangeLog b/ChangeLog
index f94e8c2b45..429fa3bdf4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Sat Jun 14 10:52:15 2014  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* string.c (rb_str_resize): update capa only when buffer get
+	  reallocated.
+	  http://d.hatena.ne.jp/nagachika/20140613/ruby_trunk_changes_46413_46420#r46413
+
 Sat Jun 14 08:28:59 2014  Zachary Scott  <e@zzak.io>
 
 	* man/rake.1: [DOC] Update links for Rake, patch by @hsbt [Bug #9904]
diff --git a/string.c b/string.c
index faf12f23e8..a8fd16ece3 100644
--- a/string.c
+++ b/string.c
@@ -1972,6 +1972,7 @@ rb_str_resize(VALUE str, long len)
     slen = RSTRING_LEN(str);
 
     {
+	long capa;
 	const int termlen = TERM_LEN(str);
 	if (STR_EMBED_P(str)) {
 	    if (len == slen) return str;
@@ -1996,11 +1997,12 @@ rb_str_resize(VALUE str, long len)
 	    if (len == slen) return str;
 	    str_make_independent_expand(str, len - slen);
 	}
-	else if (slen < len || (RSTRING(str)->as.heap.aux.capa - len) > (len < 1024 ? len : 1024)) {
+	else if ((capa = RSTRING(str)->as.heap.aux.capa) < len ||
+		 (capa - len) > (len < 1024 ? len : 1024)) {
 	    REALLOC_N(RSTRING(str)->as.heap.ptr, char, len + termlen);
+	    RSTRING(str)->as.heap.aux.capa = len;
 	}
 	else if (len == slen) return str;
-	RSTRING(str)->as.heap.aux.capa = len;
 	RSTRING(str)->as.heap.len = len;
 	TERM_FILL(RSTRING(str)->as.heap.ptr + len, termlen); /* sentinel */
     }