diff --git a/ChangeLog b/ChangeLog
index 9b9e186a09..72bcbe5c12 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu Jan 14 09:34:31 2010  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* string.c (rb_str_concat): raise RangeError when the argument is
+	  negative value. [ruby-core:27583]
+
 Thu Jan 14 08:49:59 2010  Tanaka Akira  <akr@fsij.org>
 
 	* time.c (time_to_r): convert to rational if internal representation
diff --git a/string.c b/string.c
index 29e84e3ffe..a89cca0050 100644
--- a/string.c
+++ b/string.c
@@ -1983,7 +1983,18 @@ rb_str_append(VALUE str, VALUE str2)
 VALUE
 rb_str_concat(VALUE str1, VALUE str2)
 {
-    if (FIXNUM_P(str2) || TYPE(str2) == T_BIGNUM) {
+    if (FIXNUM_P(str2)) {
+	if (NEGFIXABLE(str2))
+	    rb_raise(rb_eRangeError, "negative argument");
+    }
+    else if (TYPE(str2) == T_BIGNUM) {
+	if (!RBIGNUM_SIGN(str2))
+	    rb_raise(rb_eRangeError, "negative argument");
+    }
+    else {
+	return rb_str_append(str1, str2);
+    }
+    {
 	rb_encoding *enc = STR_ENC_GET(str1);
 	unsigned int c = NUM2UINT(str2);
 	long pos = RSTRING_LEN(str1);
@@ -1995,7 +2006,6 @@ rb_str_concat(VALUE str1, VALUE str2)
 	ENC_CODERANGE_SET(str1, cr);
 	return str1;
     }
-    return rb_str_append(str1, str2);
 }
 
 st_index_t