diff --git a/ChangeLog b/ChangeLog
index d0c0e39352..293a066e09 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Tue Oct 29 16:49:03 2013  Koichi Sasada  <ko1@atdot.net>
+
+	* gc.c (vm_malloc_increase): decrease it more carefully.
+
 Tue Oct 29 16:24:52 2013  Koichi Sasada  <ko1@atdot.net>
 
 	* gc.c (heap_page_resurrect): return a page in tomb heap even if
diff --git a/gc.c b/gc.c
index f3f6d62c4b..157a0856e5 100644
--- a/gc.c
+++ b/gc.c
@@ -5022,12 +5022,12 @@ vm_malloc_increase(rb_objspace_t *objspace, size_t new_size, size_t old_size, in
     }
     else {
 	size_t sub = old_size - new_size;
-	if (sub > 0) {
-	    if (malloc_increase > sub) {
-		ATOMIC_SIZE_SUB(malloc_increase, sub);
-	    }
-	    else {
-		malloc_increase = 0;
+	if (sub != 0) {
+	  retry_sub:;
+	    {
+		size_t old_increase = malloc_increase;
+		size_t new_increase = old_increase > sub ? old_increase - sub : 0;
+		if (ATOMIC_SIZE_CAS(malloc_increase, old_increase, new_increase) != old_increase) goto retry_sub;
 	    }
 	}
     }