Kansai International Airport is under construction in Osaka Bay, 5 km off the coast of Senshu. The construction of seawalls in the airport island commenced in January, 1987. With the opening of the airport scheduled for the spring of 1993, the construction of aboveground facilities will soon begin. Construction of this airport, designed as a key international airport in Japan, is being carried out as a national project. In Osaka Bay, land reclamation is strictly controlled by provisions in the Special Law for Environmental Conservation in the Seto Inland Sea. Accordingly, special emphasis has been placed on preventing environmental pollution in and around Osaka Bay, and conserving the natural environment in preparing the airport plan, as well as in assessing environmental impact prior to construction. This paper outlines the environmental impact assessment regarding construction of Kansai International Airport and environmental monitoring during construction, focusing on the impact on water quality.