diff --git a/ChangeLog b/ChangeLog
index c08b060138..0119c9a8f1 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Jul  6 00:34:50 2010  Yusuke Endoh  <mame@tsg.ne.jp>
+
+	* vm.c (thread_free): free altstack to prevent memory leak.  a patch
+	  from Tomoyuki Chikanaga in [ruby-dev:41815]. [Bug #3537]
+
 Tue Jul  6 00:29:27 2010  Yusuke Endoh  <mame@tsg.ne.jp>
 
 	* string.c (NONASCII_MASK): NONASCII_MASK must be unsigned.
diff --git a/vm.c b/vm.c
index dd5de1f4b7..8e93963a39 100644
--- a/vm.c
+++ b/vm.c
@@ -1726,6 +1726,11 @@ thread_free(void *ptr)
 	    RUBY_GC_INFO("main thread\n");
 	}
 	else {
+#ifdef USE_SIGALTSTACK
+	    if (th->altstack) {
+		free(th->altstack);
+	    }
+#endif
 	    ruby_xfree(ptr);
 	}
     }