This article examines the significance of Japan’s collaborative role in promoting regional integration in post-cold war East Asia, focusing on the institutionalization of the ASEAN+3 (the Association of Southeast Asian Nations+ China, Japan, and South Korea) from 1999 to 2005. This empirical case is essential to explain the main framework for the diffusion of regional institutionalization, which draws attention to Japan’s collaborative role in the development of an effective institutional arrangement in East Asia. The ASEAN+3 process is the dominant engine for the formation of an East Asian Integration Regime (EAIR). Since the Joint Statement on East Asian Cooperation in 1999, Japan has taken advantage of changing circumstances to carve out a collaborative role in promoting the institutionalization of ASEAN+3. Seen in this light, the discussion seeks to explain Japan’s regional projects in accordance with the rules and decision-making procedures of that group.