This paper describes the numerical simulation of the effect on tidal current and pollutant dispersion in the Sea of Harima and the Bay of Osaka due to an off-shore airport, which was one of the prospected location for New Kansai International Airport at Off-Takasago in the Sea of Harima.
　The area concerned covers the Strait of Bisan, the Sea of Harima, and the Bay of Osaka, which is divided into 2.5 km meshes.
　In tidal current simulation, tide is excerted by spring tide and neap one at the Strait of Bisan, the Strait of Naruto, and the Strait of Kitan.
　In water-quality simulation, the dispersion speed and the pattern of conservative substance are investigated from injection points to open sea during some cycles in case of initial concentration of zero and some given values by the steady-state analysis.
　From the comparison between the simulations of present state geographical configuration and those of future, no significant influence can be found in tidal current and pollutant dispersion in the Sea of Harima and the Bay of Osaka due to the offshore airport and/or port reclamations.