日本民主黨於2009年8月30日的眾議院大選中，以308席比119席壓倒性的席次擊敗自民黨，成為1955年以來首次單獨取得執政權的日本在野黨。民主黨從1998年創黨以來，在歷經短短的11年時間便順利取得執政權，這在日本學界興起一片「民主黨學」的研究熱潮，同時也使得外界對於這個年輕政黨在意識形態、經濟政策及外交政策與自民黨有何異同產生相當大的興趣。本文嘗試從靜態的制度面與動態的互動面兩個面向來解構日本民主黨，並進而分析民主黨的經濟與外交政策，本文主要論點認為，民主黨是一個派系橫陳的合議制政黨，它並無一個中心明確的意識形態與論述，而其外交政策也是透過內部派系之間妥協所產生的結果，這是民主黨又再次出現短命內閣的最主要因素。 Under the leadership of Yukio Hatoyama, the Democratic Party of Japan (DPJ) attained the overwhelming electoral victory on August 30, 2009, ending the one-party domination by the Liberal Democratic Party (LDP). That victory also made the DPJ the first opposition party since 1955 in Japan that defeated the incumbent ruling party by assuming an absolute majority by itself in the lower house. That the DPJ accomplished this after only eleven years since its establishment in 1998 triggered a wave of research on its ideology as well as economic and foreign policies. In accordance with this line, this article argues that Japan's lingering economic recession since the 1990s is the most crucial factor to bring about the regime change. After the election, the entrance of a large number of fresh members in the lower house of the Diet will facilitate the convergence of liberalism. These arguments will be couched in an anatomy that integrates the static institutions and the dynamics interactions among chief political forces.