This chapter aims to provide a fresh interpretation of Japan’s patterned approach to the Association of Southeast Asian Nations (ASEAN) over the past forty years, examining it in the context of Japan’s interest in leading wider concepts of regional institutions in Asia and the Pacific. It analyzes Japan’s policy direction and diplomatic efforts to maintain good relations with ASEAN as a key precondition for its commitment to establishing six different regional initiatives or institutions beyond Southeast Asia, representing a view that Japan has promoted its ASEAN policy in parallel with its commitment to wider regional institutions. Based on the analytical presumptions promoted by neoclassical realism, this chapter pursues Japanese policy responses to the international/regional structural changes through examining, especially, Japanese prime ministers’ perceptions, ideas, and roles to identify Japan’s distinctive moves on ASEAN and wider regional institutions: the end of the Vietnam War for ASEAN, the Plaza Accord and regional economic interdependence for APEC, the AFC for APT, China’s charm offensive diplomacy for EAS, China’s hegemonic rise for TPP, and Japan-China competition over economic rules for FOIP. Japan has acknowledged the solidarity and integration of ASEAN as a prerequisite for the effective development of these wider regional institutions, making it a significant task for Japan to ease ASEAN’s concern about its possible marginalization within Asia a politics and economics.