The seawater exchange is closely related to the spreading of pollutant and the water quality in coastal seas. The Seto Inland Sea is a semi-enclosed coastal sea which has a series of broad basins and narrow straits, and the along-channel outflows from the strait located at the center of the channel toward east and west may play an important role in seawater exchange. In this study, flows along the boundary between Bisan Seto which is located at the center of the Seto Inland Sea and the adjacent basin called Harima Nada were measured using a ship-mounted Acoustic Doppler Current Profiler and residual flows were obtained by removing tidal currents. The survey results showed significant volume flux of the along-channel outflows from Bisan Seto toward Harima Nada. A numerical model based on Princeton Ocean Model was also used in order to analyze the driving forces of the along-channel outflow. The outflow was well reproduced using the model which was driven by tide and horizontal density gradient between the strait and basin, suggesting that these possibly induce the seawater exchange in the Seto Inland Sea.