This article reports the following two facts with regards to the Japanese policy on the East Asian regionalism. First, the policy is severely affected by the so called ``China threat`` factor. Second, the strength of the factor is largely determined by the U.S-China relationship. For example, when the two giant countries maintain friendly and amicable relationship, the policy effect of the China threat in Japan declines or disappears. However, when the case goes the other way around, the negative effect of the China threat on the Japanese policy increases or revives. To sum up, Japan`s policy on the East Asian regionalism depends considerably on the U.S-China relationship, which defines policy implications of the China threat. In this situation, is there any room for South Korea to make influence on the Japanese policy on the East Asian regionalism? This is the question to which this article tries to answer.