diff --git a/tdiary-io-mongodb.gemspec b/tdiary-io-mongodb.gemspec
index 5815730..b902db7 100644
--- a/tdiary-io-mongodb.gemspec
+++ b/tdiary-io-mongodb.gemspec
@@ -18,6 +18,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.add_dependency "mongoid", "~> 7.0"
+  spec.add_dependency "mongo", "< 2.9.0"
   spec.add_dependency "hikidoc"
   spec.add_dependency "tdiary", ">= 5.0"