diff --git a/sisimai.gemspec b/sisimai.gemspec
index 465d7eb2..4dab1c1a 100644
--- a/sisimai.gemspec
+++ b/sisimai.gemspec
@@ -21,7 +21,7 @@ Gem::Specification.new do |spec|
 
   spec.required_ruby_version = '>= 2.1.0'
 
-  spec.add_development_dependency 'bundler', '~> 1.8'
+  spec.add_development_dependency 'bundler', '~> 2.1'
   spec.add_development_dependency 'rake', '~> 10.0'
   spec.add_development_dependency 'rspec', '~> 0'
   spec.add_runtime_dependency 'oj', '>= 3.0.0'