diff --git a/fluent-plugin-mongo.gemspec b/fluent-plugin-mongo.gemspec
index 3c7d30d..c14382d 100644
--- a/fluent-plugin-mongo.gemspec
+++ b/fluent-plugin-mongo.gemspec
@@ -17,6 +17,7 @@ Gem::Specification.new do |gem|
   gem.require_paths = ['lib']
 
   gem.add_dependency "fluentd", [">= 0.14.22", "< 2"]
+  gem.add_dependency "bigdecimal", "~> 4.0"
   gem.add_runtime_dependency "mongo", ">= 2.15.0", "< 2.19.0"
   gem.add_development_dependency "rake", ">= 0.9.2"
   gem.add_development_dependency "simplecov", ">= 0.5.4"