diff --git a/doc/guides/mrbgems.md b/doc/guides/mrbgems.md
index f75231f71082eb5348ed535b8302b31200a72fa8..258f405b5ef3996d53915ae9637eb53b5f75225a 100644
--- a/doc/guides/mrbgems.md
+++ b/doc/guides/mrbgems.md
@@ -153,6 +153,9 @@ MRuby::Gem::Specification.new('c_and_ruby_extension_example') do |spec|
 
   # Use latest mruby-onig-regexp from github. (version requirements can be omitted)
   spec.add_dependency('mruby-onig-regexp', :github => 'mattn/mruby-onig-regexp')
+
+  # You can add extra mgems active only on test
+  spec.add_test_dependency('mruby-process', :github => 'iij/mruby-process')
 end
 ```