diff --git a/README.md b/README.md
index 1ba79507a8..ef66768dc9 100644
--- a/README.md
+++ b/README.md
@@ -29,7 +29,7 @@ Fluentd: Open-Source Log Collector
 
 ### Prerequisites
 
-- Ruby 2.4 or later
+- Ruby 2.5 or later
 - git
 
 `git` should be in `PATH`. On Windows, you can use `Github for Windows` and `GitShell` for easy setup.
diff --git a/fluentd.gemspec b/fluentd.gemspec
index a5b5c805e3..022ee7ec6e 100644
--- a/fluentd.gemspec
+++ b/fluentd.gemspec
@@ -16,7 +16,7 @@ Gem::Specification.new do |gem|
   gem.require_paths = ["lib"]
   gem.license = "Apache-2.0"
 
-  gem.required_ruby_version = '>= 2.4'
+  gem.required_ruby_version = '>= 2.5'
 
   gem.add_runtime_dependency("bundler")
   gem.add_runtime_dependency("msgpack", [">= 1.3.1", "< 2.0.0"])