diff --git a/ChangeLog b/ChangeLog
index eeca7e41f7..b5df0cd2d1 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sun Nov 13 23:47:29 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* ext/dbm/extconf.rb: use convertible_int.
+
 Sun Nov 13 23:45:57 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* lib/mkmf.rb (checking_for): should not modify the result.
diff --git a/ext/dbm/dbm.c b/ext/dbm/dbm.c
index 1850b28c2f..3409d8faa0 100644
--- a/ext/dbm/dbm.c
+++ b/ext/dbm/dbm.c
@@ -21,12 +21,11 @@
 #include <fcntl.h>
 #include <errno.h>
 
-#if SIZEOF_DSIZE > SIZEOF_INT
-# define DSIZE_TYPE long
+#define DSIZE_TYPE TYPEOF_DATUM_DSIZE
+#if SIZEOF_DATUM_DSIZE > SIZEOF_INT
 # define RSTRING_DSIZE(s) RSTRING_LEN(s)
 # define TOO_LONG(n) 0
 #else
-# define DSIZE_TYPE int
 # define RSTRING_DSIZE(s) RSTRING_LENINT(s)
 # define TOO_LONG(n) ((long)(+(DSIZE_TYPE)(n)) != (n))
 #endif
diff --git a/ext/dbm/extconf.rb b/ext/dbm/extconf.rb
index c3ebe898f4..da20c5681a 100644
--- a/ext/dbm/extconf.rb
+++ b/ext/dbm/extconf.rb
@@ -71,12 +71,6 @@ if dblib.any? {|db| headers.fetch(db, ["ndbm.h"]).any? {|hdr| headers.db_check(d
   have_header("sys/cdefs.h")
   have_func("dbm_pagfno((DBM *)0)", headers.found, headers.defs)
   have_func("dbm_dirfno((DBM *)0)", headers.found, headers.defs)
-  type = checking_for "sizeof(datum.dsize)", STRING_OR_FAILED_FORMAT do
-    pre = headers.found + [["static datum conftest_key;"]]
-    %w[int long LONG_LONG].find do |t|
-      try_static_assert("sizeof(conftest_key.dsize) <= sizeof(#{t})", pre, headers.defs)
-    end
-  end
-  $defs << "-DSIZEOF_DSIZE=SIZEOF_"+type.tr_cpp if type
+  convertible_int("datum.dsize", headers.found, headers.defs)
   create_makefile("dbm")
 end