Purification of polluted sea water in deep bays is essential from the viewpoints of coastal environment. Development of artificial fishing grounds is also desirable from the standpoint of coast fisheries. These two social problems need commonly the development of technology to generate net flow in coastal region and to control it. We are demonstrating how to gain the difference of water head by overtopping waves into a coastal structure, and how to discharge the overtopped sea water into a bay or deep sea by using the head difference. This study is concerned with the maximum quantity of wave overtopping gained when two-dimensional regular waves climb up a sloping bed. The maximum quantity can be evaluated by using non-dimensional empirical diagrams.