diff --git a/ext/digest/extconf.rb b/ext/digest/extconf.rb
index ba2c7b671f..0e5f08d794 100644
--- a/ext/digest/extconf.rb
+++ b/ext/digest/extconf.rb
@@ -4,4 +4,8 @@
 
 require "mkmf"
 
+$INSTALLFILES = {
+  "digest.h" => "$(HDRDIR)"
+} if $extmk
+
 create_makefile("digest")