diff --git a/.gitignore b/.gitignore
index 9106b2a..dc96cb9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,3 +6,4 @@
 /pkg/
 /spec/reports/
 /tmp/
+/Gemfile.lock
diff --git a/Gemfile b/Gemfile
index eb86192..b4e2a20 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,4 +1,3 @@
 source "https://rubygems.org"
 
-gem "rake"
-gem "test-unit"
+gemspec
diff --git a/Gemfile.lock b/Gemfile.lock
deleted file mode 100644
index 98180a7..0000000
--- a/Gemfile.lock
+++ /dev/null
@@ -1,17 +0,0 @@
-GEM
-  remote: https://rubygems.org/
-  specs:
-    power_assert (1.2.0)
-    rake (13.0.1)
-    test-unit (3.3.5)
-      power_assert
-
-PLATFORMS
-  ruby
-
-DEPENDENCIES
-  rake
-  test-unit
-
-BUNDLED WITH
-   2.2.0.dev
diff --git a/optparse.gemspec b/optparse.gemspec
index afb9042..e62f094 100644
--- a/optparse.gemspec
+++ b/optparse.gemspec
@@ -28,4 +28,7 @@ Gem::Specification.new do |spec|
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{\Aexe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
+
+  spec.add_development_dependency "rake", "~> 13.0"
+  spec.add_development_dependency "test-unit", "~> 3.4"
 end