diff --git a/fluent-plugin-s3.gemspec b/fluent-plugin-s3.gemspec
index 7ee937c..f1310c8 100644
--- a/fluent-plugin-s3.gemspec
+++ b/fluent-plugin-s3.gemspec
@@ -23,6 +23,7 @@ Gem::Specification.new do |gem|
   gem.add_development_dependency "test-unit", ">= 3.0.8"
   gem.add_development_dependency "test-unit-rr", ">= 1.0.3"
   gem.add_development_dependency "timecop"
+  gem.add_development_dependency "ostruct"
   # aws-sdk-core requires one of ox, oga, libxml, nokogiri or rexml,
   # and rexml is no longer default gem as of Ruby 3.0.
   gem.add_development_dependency "rexml"