East Asian countries, for example "ASEAN plus three countries" (China, Korea, and Japan), have been well cognizant of importance of the regional financial cooperation since the Asian currency crisis in 1997. They have established the Chiang Mai Initiative (CMI) to manage currency crises. However, the CMI is not designed for "crisis prevention" because it includes no more than soft surveillance process as well as a network of currency swap arrangements. The surveillance process should be conducted over intra-regional exchange rates and exchange rate policies of the regional countries in order to stabilize intra-regional exchange rates in a situation of a strong economic relationship among the regional countries. On one hand, the regional exchange rate stability is related with an optimum currency area. Based on a Generalized PPP model, which detects a cointegration relationship among real effective exchanges rates, we investigate whether the region composed of "ASEAN plus three countries" is an optimum currency area. In the investigation, our interest is focused on an issue whether the Japanese yen could be regarded as an "insider" currency as well as other East Asian currencies. Or, is the Japanese yen still an "outsider" which is used as a target currency of foreign exchange rate policy for other East Asian countries. We employ a Dynamic OLS to estimate the long-term relationship among the East Asian currencies in a currency basket. Our empirical results indicate that the Japanese yen works as an exogenous variable in the cointegration system during a pre-crisis period while it works as an endogenous one during a post-crisis period. It implies that the Japanese yen could be regarded as an insider currency as well as other East Asian currencies after the crisis although it is regarded as an outsider currency as well as the US dollar and the euro before the Asian crisis.