diff --git a/extconf.rb b/extconf.rb
index 743b1eb..aff37e4 100644
--- a/extconf.rb
+++ b/extconf.rb
@@ -1,3 +1,4 @@
+require 'rbconfig'
 mf = File.read('Makefile')
 mf = mf.gsub(/^BINDIR\s*=.*$/, "BINDIR = #{RbConfig::CONFIG['bindir']}")
 mf = mf.gsub(/^PREFIX\s*=.*$/, "PREFIX = #{File.dirname(RbConfig::CONFIG['libdir'])}")