日本在九○年代推動政治改造工程，先後完成選舉制度改革與行政改革，分別強化了政黨總裁與首相的政治權力。小泉純一郎當選自民黨總裁並順利組閣執政後，充分地運用了政治改造工程賦予的權限，削弱了派閥對政治的影響力，改變了既往的政治決定模式，「小泉政治」成為現代日本政治的研究焦點，而郵政民營化的推動便是最好的例子。本文將從小泉首相推動郵政民營化的過程中，分析在制度改革下，日本首相領導權的強化。

Japan started a political reconstruction project at 1990s and now has accomplished two reforms: One is ''electoral system reform”, which consolidates the party president's political power; and the other is ''administrative reform”, which strengthens Japan prime minister's political authority. After holding the LDP's presidential position and forming a cabinet, Koizumi thoroughly utilized the authority that political reconstruction project granted. He weakened the influences of factions and rectified the policy-making pattern. Consequently, ''Koizumi Regime'' becomes the focus of modern Japan politics researches, and the promotion of postal privatization bill is an absorbing case. The aim of this paper is to analyze the transformations of policy-making pattern from the process of postal privatization bill promoted by Prime Minister Koizumi.