diff --git a/ChangeLog b/ChangeLog
index 110aaddf88..92b01f9e6a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri May 18 17:25:33 2012  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* cont.c: bump up fiber machine stack size when running on 64bit
+	  arch. [Bug #6344] [ruby-dev:45554]
+
 Fri May 18 15:20:56 2012  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* lib/uri/generic.rb (URI::Generic.build): duplicate args before adding
diff --git a/cont.c b/cont.c
index 633900fff8..839b6703a9 100644
--- a/cont.c
+++ b/cont.c
@@ -47,8 +47,13 @@
 #define RB_PAGE_SIZE (pagesize)
 #define RB_PAGE_MASK (~(RB_PAGE_SIZE - 1))
 static long pagesize;
-#define FIBER_MACHINE_STACK_ALLOCATION_SIZE  (0x10000)
-#endif
+
+ #if SIZEOF_VOIDP==8
+  #define FIBER_MACHINE_STACK_ALLOCATION_SIZE  (0x100000)
+ #else
+  #define FIBER_MACHINE_STACK_ALLOCATION_SIZE  (0x10000)
+ #endif
+#endif /*FIBER_USE_NATIVE*/
 
 #define CAPTURE_JUST_VALID_VM_STACK 1