diff --git a/ChangeLog b/ChangeLog
index 83de327daf..4dacc2a872 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Fri May  8 10:07:02 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* array.c (rb_ary_flatten_bang): clears temporary array.
+
 Fri May  8 02:30:14 2009  Akinori MUSHA  <knu@iDaemons.org>
 
 	* lib/set.rb (SortedSet#add): Do not require each newly added
diff --git a/array.c b/array.c
index 034aa7fcf0..ece53ee68c 100644
--- a/array.c
+++ b/array.c
@@ -382,10 +382,18 @@ void
 rb_ary_free(VALUE ary)
 {
     if (ARY_OWNS_HEAP_P(ary)) {
-	xfree(RARRAY_PTR(ary));
+	xfree(ARY_HEAP_PTR(ary));
     }
 }
 
+static inline void
+ary_discard(VALUE ary)
+{
+    rb_ary_free(ary);
+    RBASIC(ary)->flags |= RARRAY_EMBED_FLAG;
+    RBASIC(ary)->flags &= ~RARRAY_EMBED_LEN_MASK;
+}
+
 static VALUE
 ary_make_shared(VALUE ary)
 {
@@ -3341,8 +3349,13 @@ rb_ary_flatten_bang(int argc, VALUE *argv, VALUE ary)
     if (level == 0) return Qnil;
 
     result = flatten(ary, level, &mod);
-    if (mod == 0) return Qnil;
+    if (mod == 0) {
+	ary_discard(result);
+	return Qnil;
+    }
+    if (!(mod = ARY_EMBED_P(result))) rb_obj_freeze(result);
     rb_ary_replace(ary, result);
+    if (mod) ARY_SET_EMBED_LEN(result, 0);
 
     return ary;
 }