This study provides an analytical framework for examining the role of the prime minister in Japan's political decision making. It shows that two dimensions of division within the government and the ruling party - interagency rivalry and intraparty factions - confront the prime minister whenever a major policy issue needs to be resolved. Depite these obstacles, it shows that the prime minister can be effective. First, the author identifies the sources of power available to Japanese prime ministers - some from legal authorities and others from informal sources. Because prime ministers must rely on informal sources of power to effectively utilize institutional sources of power, their effectiveness varies depending on their background, experience, political skills and personality. Shinoda identifies six major informal sources of power: power base within the ruling party, control over the bureaucracy, ties with the opposition parties, public support, business support and international reputation. The head of government's leadership style can be defined according to which sources of power they utilize in the policy process. The author presents both successful and unsuccessful case studies - Hashimoto's administrative reform, Takeshita's tax reform and Nakasone's administrative reform - illustrating how different prime ministers have succeeded or failed in applying their political resources. After examining these three case studies, he uncovers four types of leadership among Japanese prime ministers. This study should be a useful analytical resource for scholars and students of Japanese politics and political economy and comparative politics.