The Constitution of Japan stands for pacifism. The Article IX in the Japanese Constitution aspires not only sincerely to an international peace based on justice and order, the Japanese people forever renounce war as a sovereign right of the nation, and the threat or use of force as means of settling international disputes. But also in order to accomplish the aim of the preceding Articles, Japan’s ground, naval, and air forces, as well as other war potential, will never be maintained. The right of belligerency of the state will not be recognized. As the result, Japanese Self-Defense Forces are mainly to defense, strategic offensive weapons have been prohibited. Japanese Self-Defense Forces is prohibited left the territory to overseas operations. Nonetheless, Japan actually has very powerful defense capabilities. The author will examine firstly whether the status is constitutional or unconstitutional, and investigate if the reality is inconsistent with the constitutional norms. In such case, how does Japanese Government explain this contradiction? This study attempts to explore these arguments according to the Constitution and the basic theory of international law. Furthermore, Japan’s established unarmed policy has to be changed due to the rapidly changing international situation, especially the situation in Northeast Asia .Under such circumstance, this research will provide theoretical analysis and reviews responding to how Japan adjust the self-constrained and self-defense principles and policies as well as the possible projection of Japan-US Security Treaty and Japan-US security system in the future. Finally, the conclusion will focus on two critical issues: whether Japan will advocate the right of collective self-defense and whether Japan will take part with UN military sanction action.