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In Okinawa, Japan, the Universal Soil Loss Equation (USLE,) has been widely used as the prediction method for red soil runoff, and the parameters of the USLE, which is suitable for the Okinawa area, have been researched by Okinawa Prefectural Institute of Health and Environment. However, the USLE is apt to overestimate measured values. This is because the USLE does not calculate the amount of runoff but erosion. The Modified Universal Soil Loss Equation (MUSLE) was developed to solve the problem. Therefore, we tried to apply the MUSLE to the Todoroki watershed (Figure a) in Ishigaki, Okinawa and examined the effectiveness of the model for the prediction of red soil runoff in this study. In the application of the MULSE, we used the method to estimate the changing of the curve number adopted in the SWAT, and calculated the changing of soil moisture in the watershed by the simple storage model. As a result, we recognized that the MUSLE estimates better than USLE which has been used in Okinawa by considering runoff rate. As further research, we have to apply to other watersheds and develop the model to calculate the CN changing.