Watanabe et al. (2003) show that the temperature of the deep water in the northern part of East/Japan Sea (EJS) oscillates with 20 year cycle as the North Pacific Index, although the intermediate to deep circulation of EJS is independent of that in the North Pacific. This suggests that the interdecadal scale variation in the EJS is caused by external factors such as the atmospheric conditions and the Tsushima warm current (TWC). Using a four-box model based on Stommel (1961) with an inflow and outflow system, we have investigated the roles of the external forcing in the interdecadal scale variation of the EJS thermohaline circulation. The model shows that the effect of TWC is manifested more strongly in the northern boxes where watermass formation occurs than the southern boxes, although the southern surface box is connected to TWC directly. The effect of the surface boundary conditions can be observed in all surface boxes, however. The model also shows TWC could have strong effect on the deep water mass formation rate.