diff --git a/fluent-plugin-prometheus.gemspec b/fluent-plugin-prometheus.gemspec
index e72ad9a..986a612 100644
--- a/fluent-plugin-prometheus.gemspec
+++ b/fluent-plugin-prometheus.gemspec
@@ -14,7 +14,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.add_dependency "fluentd", ">= 0.14.20", "< 2"
-  spec.add_dependency "prometheus-client"
+  spec.add_dependency "prometheus-client", "< 0.10"
   spec.add_development_dependency "bundler"
   spec.add_development_dependency "rake"
   spec.add_development_dependency "rspec"