diff --git a/.rubocop.yml b/.rubocop.yml
index ae465c0..5a1407d 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -1,5 +1,5 @@
 AllCops:
-  TargetRubyVersion: 2.3
+  TargetRubyVersion: 2.4
   # RuboCop has a bunch of cops enabled by default. This setting tells RuboCop
   # to ignore them, so only the ones explicitly set in this file are enabled.
   DisabledByDefault: true
diff --git a/.travis.yml b/.travis.yml
index 237ef94..f471edf 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -25,8 +25,6 @@ matrix:
       gemfile: gemfiles/rails52.gemfile
     - rvm: 2.4
       gemfile: gemfiles/rails52.gemfile
-    - rvm: 2.3
-      gemfile: gemfiles/rails52.gemfile
     - rvm: 2.5
       gemfile: gemfiles/rails51.gemfile
     - rvm: 2.4
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 57dee0d..f3acab4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,8 @@
 
 ## master
 
+- **Ruby 2.4+** is required. 
+
 - Fix Rails 6 compatibility. ([@palkan][])
 
 ## 0.5.0 (2019-02-24)
diff --git a/action-cable-testing.gemspec b/action-cable-testing.gemspec
index 8db4aea..857f1a4 100644
--- a/action-cable-testing.gemspec
+++ b/action-cable-testing.gemspec
@@ -22,7 +22,7 @@ Gem::Specification.new do |spec|
 
   spec.require_paths = ["lib"]
 
-  spec.required_ruby_version = ">= 2.3.0"
+  spec.required_ruby_version = ">= 2.4.0"
 
   spec.add_dependency "actioncable", ">= 5.0"