This thesis explores changes in Japan's conception of and policy toward security multilateralism in the Asia-Pacific region after the end of the Cold War with special reference to the ASEAN Regional Forum (ARF). To understand the complex processes behind the changes in and formation of these perceptions, this study develops an eclectic analysis, which draws on the insights of various theoretical models in International Relations (IR). Based on a persistent analysis of Japan's diplomacy in security institution building over the decade, this study goes on not only to illuminate the problems and the direction of Japan's post-Cold War security policy but also to provide an empirical basis for examining the validity of three theoretical perspectives on the role and efficacies of security institutions, namely realism, neoliberal institutionalism and constructivism. This thesis makes the observation that notwithstanding strong multilateral impulses arising in Japanese policy thinking in the early 1990s, Japan's enthusiasm for regional security multilateralism has dwindled since the late 1990s. It argues that this has been mainly due to Japan's disappointing experiences in the ARF, evinced by abortive efforts to promote cooperative security measures and the failings of multilateral security diplomacy in addressing Japan's security concerns. Japan's enthusiasm for regional security multilateralism has been further diluted by a number of domestic and international constraints on the ARF policy. Combined with perceived limitations of the effectiveness of the ARF, these factors have made Japanese policymakers more inclined to devote their energies to the military dimension of the nation's security policy, in particular its defence cooperation with the USA. Consequently, Japanese conceptions of the ARF have been shifted from an optimistic liberal to a more pessimistic realist perspective, from which it can at best be seen to contribute to making the existing bilateral alliance arrangements in the region work more smoothly