diff --git a/Gemfile b/Gemfile
index 4f1a720..fb35b7e 100644
--- a/Gemfile
+++ b/Gemfile
@@ -7,6 +7,11 @@ group :development do
   gem "bundler"
   gem "rake"
   gem "rdoc"
+
+  # rdoc 8 depends on rbs, which has no java platform gem before 4.1.0.pre.2.
+  # See https://github.com/ruby/rdoc/issues/1746
+  gem 'rbs', '>= 4.1.0.pre.2' if RUBY_PLATFORM == 'java'
+
   gem "test-unit"
   gem "test-unit-ruby-core"
   gem 'rake-compiler'