Party alternation is an important condition for the development of modern democracy. Although the Japanese democracy after World War II proceeded from the enforcement of the new constitution, LDP dominated the Japanese politics since 1955. The party alternation thinking in Japan was limited by the situations of domestic and international environment such as politics, economics, society and parties. The end of the Cold War liberated the party alternation thinking in Japan, and led to the formation and development of DPJ. This paper aims at the changes of party alternation thinking in Japan based upon the theories of democracy, as well as the aspect of Japanese political development. Furthermore, the roles of DPJ in the development of party alternation thinking in Japan were also proposed in this paper.