diff --git a/ext/groonga/extconf.rb b/ext/groonga/extconf.rb
index 3b6426a7..1f00ae2d 100755
--- a/ext/groonga/extconf.rb
+++ b/ext/groonga/extconf.rb
@@ -29,7 +29,7 @@
   require "pkg-config"
 end
 
-base_dir = Pathname(__FILE__).dirname.parent.parent
+base_dir = Pathname(__FILE__).dirname.parent.parent.expand_path
 $LOAD_PATH.unshift(base_dir.to_s)
 
 require "rroonga-build"