This article employs a simple model of sophisticated voting under incomplete information and explores the strategic contexts of the vote on political reform bills in Japan. The government-sponsored political reform bills were voted down by the defection of government coalition members in the House of Councillors before a final compromise was reached in the joint committee of both houses and passed subsequently. In contrast to the accepted view that the defectors were short-sighted sincere voters, I show that Japan's institutional arrangements created an uncertainty about the agenda in the legislative process and led to the sophisticated voting behavior of pivotal voters whose preferences were different from the party leadership. The analysis underscores the importance of sophisticated voting for the empirical study of Japanese legislative politics.