This paper describes the investigation of the mechanism of water exchange with density current by hydraulic model experiments.
　In enclosed inner-bay, the water exchange is caused mainly by verical circulation due to density current,horizontal circulation due to tidal residual flow, and turbulent diffusion due to tidal current. The influence of marine structures on the water exchange in the coastal region is investigated by physical model and numerical simulation techniques.
　In order to reproduce density current, seawater and freshwater are used in the hydraulic model experiments. From the experiments, the comparisons of the water exchange behaviors with and without density difference are carried out. The characteristics of the water exchange between inner and outer bay waters are investigated by the hydraulic model experiments and the numerical simulations.
　From these results, it is pointed out that the vertical circulation caused by density current has a large effect on exchange waters in enclosed regions. The experimental method described in the report is useful toconsider the water exchange mechanism in stratified water conditions.