This research aims at exploring the influence of the United States upon the development of "regional arrangement complex" in Asia Pacific, and at identifying the implications of developing such "complex" on the relationship between the United States and Japan. Specifically this research project will (1) identify what were US interests in regional organizations (proposed or existing), (2) analyze how, pursuing its interests, US power and influence affected the goal and activities of respective organizations, (3) examine how institutional norms and ideas limited US power and influence and (4) clarify how development of "regional arrangement complex" in Asia Pacific has affected and will affect the US-Japan relationship in the future. Specifically, such critical cases will be chosen as East Asian Economic Group proposed by Malaysia in 1990, ARF established in 1994, accelerated trade liberalization in APEC proposed by the United States in the mid 1990s, Asian Monetary Fund proposed by Japan in 1997, and APT institutionalized in 1997, SCO and ACD was established in the early 2000s, various FTA were/have been promoted, and so on. Through those case studies, the US factor in "regional arrangement complex" will be clarified . In this research, the newly developed concept of "regional arrangement complex" will be introduced in order to describe the situation where various arrangements, either governmental or nongovernmental, overlap with one another so that they are mutually interrelated in a certain geographical sphere. Today, there are various regional arrangements, each of which embodies and reflects respective regionalism, and "regional arrangement complex" has been developing especially after the Cold War. The United States is the key player in such "regional arrangement complex" in Asia Pacific. US power and influence should be examined carefully because within "regional arrangement complex" in Asia Pacific, the US Government is ambivalent about, and also probably frustrated by, American position due to the way the United States is involved in the complex. US power is crucial in maintaining regional order in Asia Pacific, and "regional arrangement complex" in Asia Pacific has been promoting regional peace and prosperity . At the same time, however, the United States turned out neither the "Almighty God" nor the "Global Empire" in the course of development of "regional arrangement complex" in the region. This research hypothesizes that US influence on regionalism in Asia Pacific is varied depending not only on its interests, but also on institution, issue and idea. Careful examination of US power upon regional arrangement complex in Asia Pacific and of US policy toward it is needed in order to understand security and economic relations in the post-Cold War era. Because the development of "regional arrangement complex" has been influenced by both Japan and the United States, it is almost self-evident that the relationship between Japan and the United States has been related to those two nations involvement in Asia Pacific regional institutions. The US-Japan relationship, one of the important pillars in Asia Pacific architecture, has determined, and has been determined conversely by, the growing "regional arrangement complex".