This study provides an analytical framework for the prime minister's role in Japan's decision-making. Two dimensions of fraction exist within the government and the ruling party, inter-agency rivalry and intra-party factions, with which the leader must confront when pursuing a major policy. Despite these obstacles the prime minister has at his disposal various sources of power with which he can play an imperative role to achieve a major policy goal. His leadership style can be defined, depending on what kind of sources of power they utilise in the policy process. After examining these three cases -- Hashimoto's administrative reform, Takeshita's tax reform and Nakasone's administrative reform -- this study introduces four types of leadership: the political insider, the grandstander, the kamikaze fighter and the peace lover.