diff --git a/gc/mmtk/extconf.rb b/gc/mmtk/extconf.rb
index b2cd9401f0..5e77da99a9 100644
--- a/gc/mmtk/extconf.rb
+++ b/gc/mmtk/extconf.rb
@@ -32,6 +32,12 @@ makefile << <<~'MAKEFILE'
   			exit 1 \
   			;; \
   	esac
+
+  clean: clean-mmtk
+
+  .PHONY: clean-mmtk
+  clean-mmtk:
+		-$(Q)$(RM_RF) debug release
 MAKEFILE
 
 File.open("Makefile", "w") { |file| file.puts(makefile) }