Japan has been important and irreplaceable for Asia-Pacific security affairs. The basic structure of Japan's defense policy was greatly regulated by ”The Basis of Japan's Defense Policy.” Whenever The Basis of Japan's Defense Policy was amended, the reports which based on the discussion held by the Japanese government between experts and scholars, as said the ”Advisory Group on Defense Issues,” will be taken as an important reference. In this manner, Advisory Group on Defense Issues puts great influence on formulation of Japan's defense policy and development of U.S.-Japan security alliance.This article aims at the reports which published by Advisory Group on Defense Issues as basic research material, and systematically analyzes the assessments of the security environment Japan situated, Japan's response strategies, and the changes and developments U.S.-Japan security alliance. This article aims to understand the context of Japan's Defense Policy and U.S.-Japan security alliance; accordingly, may provide references for Taiwan's security.