diff --git a/ChangeLog b/ChangeLog
index a8abbe6de5..9c900075b5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+Sat Dec 22 13:15:08 2012  Hiroshi Shirosaki  <h.shirosaki@gmail.com>
+
+	* gc.c (obj_id_to_ref): add a macro to treat Bignum object id.
+	  This follows the change r38493.
+
+	* gc.c (id2ref): fix for working fine with Bignum object id on x64
+	  Windows.
+	* gc.c (wmap_finalize): ditto.
+
 Sat Dec 22 11:30:21 2012  Masaki Matsushita  <glass.saga@gmail.com>
 
 	* struct.c (make_struct): remove junk ID check to allow members who
diff --git a/gc.c b/gc.c
index 217d23be4c..6133fe5c6c 100644
--- a/gc.c
+++ b/gc.c
@@ -292,8 +292,11 @@ int *ruby_initial_gc_stress_ptr = &rb_objspace.gc_stress;
 
 #if SIZEOF_LONG == SIZEOF_VOIDP
 # define nonspecial_obj_id(obj) (VALUE)((SIGNED_VALUE)(obj)|FIXNUM_FLAG)
+# define obj_id_to_ref(objid) ((objid) ^ FIXNUM_FLAG) /* unset FIXNUM_FLAG */
 #elif SIZEOF_LONG_LONG == SIZEOF_VOIDP
 # define nonspecial_obj_id(obj) LL2NUM((SIGNED_VALUE)(obj) / 2)
+# define obj_id_to_ref(objid) (FIXNUM_P(objid) ? \
+   ((objid) ^ FIXNUM_FLAG) : (NUM2PTR(objid) << 1))
 #else
 # error not supported
 #endif
@@ -1630,7 +1633,7 @@ id2ref(VALUE obj, VALUE objid)
     if (ptr == Qnil) return Qnil;
     if (FIXNUM_P(ptr)) return (VALUE)ptr;
     if (FLONUM_P(ptr)) return (VALUE)ptr;
-    ptr = objid ^ FIXNUM_FLAG;	/* unset FIXNUM_FLAG */
+    ptr = obj_id_to_ref(objid);
 
     if ((ptr % sizeof(RVALUE)) == (4 << 2)) {
         ID symid = ptr / sizeof(RVALUE);
@@ -3795,7 +3798,7 @@ wmap_finalize(VALUE self, VALUE objid)
 
     TypedData_Get_Struct(self, struct weakmap, &weakmap_type, w);
     /* Get reference from object id. */
-    obj = objid ^ FIXNUM_FLAG; /* unset FIXNUM_FLAG */
+    obj = obj_id_to_ref(objid);
 
     /* obj is original referenced object and/or weak reference. */
     orig = (st_data_t)obj;