diff --git a/ChangeLog b/ChangeLog
index dde8448c5d..3deb996f41 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri Jul 19 15:32:57 2013  Koichi Sasada  <ko1@atdot.net>
+
+	* array.c (rb_ary_store): use RARRAY_PTR_USE() intead of RARRAY_PTR().
+	  Clearing memory space doesn't need WBs.
+
 Fri Jul 19 15:19:37 2013  Koichi Sasada  <ko1@atdot.net>
 
 	* array.c (ary_ensure_room_for_push): use RARRAY_RAWPTR() instead of
diff --git a/array.c b/array.c
index 46f37944a0..f3f1b2e960 100644
--- a/array.c
+++ b/array.c
@@ -797,8 +797,10 @@ rb_ary_store(VALUE ary, long idx, VALUE val)
 	ary_double_capa(ary, idx);
     }
     if (idx > RARRAY_LEN(ary)) {
-	rb_mem_clear(RARRAY_PTR(ary) + RARRAY_LEN(ary),
+	RARRAY_PTR_USE(ary, ptr, {
+	    rb_mem_clear(ptr + RARRAY_LEN(ary),
 		     idx-RARRAY_LEN(ary) + 1);
+	});
     }
 
     if (idx >= RARRAY_LEN(ary)) {