diff --git a/.ruby-version b/.ruby-version
new file mode 100644
index 0000000..58073ef
--- /dev/null
+++ b/.ruby-version
@@ -0,0 +1 @@
+2.4.1
\ No newline at end of file
diff --git a/normal_distribution.gemspec b/normal_distribution.gemspec
index 73a2229..9b4c0cb 100644
--- a/normal_distribution.gemspec
+++ b/normal_distribution.gemspec
@@ -29,11 +29,12 @@ Gem::Specification.new do |spec|
   spec.files         = Dir.chdir(File.expand_path('..', __FILE__)) do
     `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
   end
-  spec.bindir        = "exe"
-  spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
-  spec.require_paths = ["lib", "ext"]
-  spec.extensions    = ["ext/normal_distribution/extconf.rb"]
-  spec.files         = Dir["lib/**/*.rb", "ext/**/*.{c,h,rb}"]
+  spec.bindir                = "exe"
+  spec.executables           = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
+  spec.require_paths         = ["lib", "ext"]
+  spec.extensions            = ["ext/normal_distribution/extconf.rb"]
+  spec.files                 = Dir["lib/**/*.rb", "ext/**/*.{c,h,rb}"]
+  spec.required_ruby_version = '>= 2.4.1'
 
   spec.add_development_dependency "bundler", "~> 1.16"
   spec.add_development_dependency "rake", "~> 10.0"