This paper aims to account for the formation of an interstate system in East Asia in the 1990s. International trading system in one form or another has continued to exist in the area since the medieval and especially modern eras. In contrast, we have had no autonomous interstate system until very recently. Before the Second World War, most of the Asian nations were Western or Japanese colonies, and thus they did not have even a fragment of their own international political system. But now all the nations in Asia are independent, and in addition, they have established their own way of political interactions among themselves. In this paper the author gathers data on changes in the diplomatic relationships among countries in the area after the end of the Second World War to probe into the systemic formation. His findings are that an autonomous interstate system has been at work in East Asia since the 1990s. Southeast Asian countries have played a leading part in forming it since the mid-1970s. Although China during the Great Cultural Revolution, and Vietnam after unification were once revolutionary, hence major turbulent actors, now both have become normal states in diplomatic terms. The end of the Cold-War in the early 1990s brought about the global milieu favorable to the formation of the interstate system in East Asia. Besides, North Korea does not look to be so turbulent as generally said, as it seems to be seeking more favorable international settings. Finally, Japanese specialists on Asia regard an Asian region as containing local international or world systems, each of which, in their views, possesses an integrity but whose function is flexible, multiple and/or overlapping. In this regard, the interstate system in East Asia is judged to be a case, although its functions are formal, basic and fixed within its range.