The invention relates to an electric power generation system for a water storage tank and a storage structure system for a water storage tank wherein the electric power generation system for the water storage tank according to the invention comprises: a water storage tank for storing rainwater or water underground for a specific purpose; an electricity generation part for generating electricity by utilizing water stored in the water storage tank; a cooling part for cooling heat generated at the time of generating electricity by the electricity generation part; and a cooling water supply part for supplying the cooling part with the water stored in the water storage tank. Thereby, dispersed electricity generation facilities can be secured by utilizing open underground spaces of the water storage tank.