A distributed runoff model was constructed for the Seto Inland Sea, a semi-enclosed sea, to quantitatively evaluate the effects of changes in rainfall patterns and land use on freshwater inflow into each sea over the past 30 years. The results showed no significant linear trend in annual precipitation and freshwater inflow. On the other hand, short-term runoff due to heavy rainfall increased in all seas after around 2010, especially in the eastern areas such as Osaka Bay, Harima-nada, and Bisan Seto. With the change in land use, the discharge and runoff rate increased in all seas, especially in the central area (Aki-nada, Hiuchi-nada, and Bingo-nada) where the urbanization rate was high, and the discharge increased by about 10 %. These results suggest that it is important to consider not only changes in rainfall patterns associated with climate change but also changes in land use when predicting future flow and water quality in the Seto Inland Sea.