PURPOSE: To construct the ground areas for fresh-water lake, reclamation site, etc., in addition to offering of conveniences by power station, etc., by constructing a dam and a pumped storage type power station in an adequate place, e.g., bay, inland sea, etc.

CONSTITUTION: A dam D is constructed in an adequate place in the neighborhood or inside of the entrance of bay, inlsnd sea, etc., and gouped into the outside G1 and inside G2 of the dam. Then, in an adequate place between the outside and inside G1 and G2, a water-pumping type power station P is constructed. The outside G1 of the dam is readjusted for an upper water reservoir R1 and the inside G2 of the sam is readjusted for a lower water reservoir R'1 (or fresh- water lake, reclamation site, etc.). By this, for usual precipitation, the power station P is put into an usual operation mode, whereas for unusual precipitation, the power station is put into a continuous full-load water-pumping operation in such a way that the lower reservoir R'1 is not filled up.