diff --git a/fluent-plugin-prometheus.gemspec b/fluent-plugin-prometheus.gemspec
index 6cb5d97..e5e0051 100644
--- a/fluent-plugin-prometheus.gemspec
+++ b/fluent-plugin-prometheus.gemspec
@@ -1,6 +1,6 @@
 Gem::Specification.new do |spec|
   spec.name          = "fluent-plugin-prometheus"
-  spec.version       = "1.3.0"
+  spec.version       = "1.4.0"
   spec.authors       = ["Masahiro Sano"]
   spec.email         = ["sabottenda@gmail.com"]
   spec.summary       = %q{A fluent plugin that collects metrics and exposes for Prometheus.}