diff --git a/red-arrow-nmatrix.gemspec b/red-arrow-nmatrix.gemspec
index 98e3847..101a578 100644
--- a/red-arrow-nmatrix.gemspec
+++ b/red-arrow-nmatrix.gemspec
@@ -38,6 +38,7 @@ Gem::Specification.new do |spec|
   spec.files = ["README.md", "Rakefile", "Gemfile", "#{spec.name}.gemspec"]
   spec.files += [".yardopts"]
   spec.files += Dir.glob("lib/**/*.rb")
+  spec.files += Dir.glob("ext/**/*.c")
   spec.files += Dir.glob("doc/text/*")
   spec.extensions = ["ext/arrow-nmatrix/extconf.rb"]
   spec.test_files += Dir.glob("test/**/*")