diff --git a/ChangeLog b/ChangeLog
index a23b0e4868..ceaee71160 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Tue Dec 25 14:20:13 2007  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* lib/mkmf.rb (map_dir): should generate path including $top_srcdir.
+
 Tue Dec 25 14:09:16 2007  James Edward Gray II  <jeg2@ruby-lang.org>
 
 	* lib/csv.rb:  Fixed test failures caused by changes to Ruby.
diff --git a/lib/mkmf.rb b/lib/mkmf.rb
index 2038f5f4cd..bb583c9add 100644
--- a/lib/mkmf.rb
+++ b/lib/mkmf.rb
@@ -152,7 +152,7 @@ if not $extmk and File.exist?(($hdrdir = RbConfig::CONFIG["rubyhdrdir"]) + "/rub
   $top_srcdir = $hdrdir
   $arch_hdrdir = $hdrdir + "/$(arch)"
 elsif File.exist?(($hdrdir = ($top_srcdir ||= topdir) + "/include")  + "/ruby.h") and
-    File.exist?("#{CONFIG["EXTOUT"]}/include/#{CONFIG["arch"]}/ruby/config.h")
+    File.exist?("#{$top_srcdir}/#{CONFIG["EXTOUT"]}/include/#{CONFIG["arch"]}/ruby/config.h")
   $topdir ||= RbConfig::CONFIG["topdir"]
   $arch_hdrdir = "$(extout)/include/$(arch)"
 else