The US-Japan Defense Cooperation is now the only security mechanism in Asia-Pacific region. The US has signed 'the Guidelines for the US-Japan Cooperation Defense' with Japan in November, 1978, which established the basic structure of cooperation defense. Despite the fact that the structure was established before the Cold War, it remained to be influential on the security of Asia-Pacific region during the post Cold-War period. The Taiwan Strait Crisis in1996 has been a significant example.To understand the formation of the US-Japan Defense Cooperation and the role of Japan's government on the Asia Pacific military affairs, the thesis deals with the following three issues. The first is to investigate the environmental change of Japan's national an international security. The second issue is to analyze the different concepts between US and Japan on 'threat' and 'war.' The last will discuss the consequence of the compromise between US and Japan, which as well involves the basic structure of the formation of the US-Japan Defense Cooperation.