To effectively use sunlight in a sunlight utilizing system utilizing a water tank.

In this system, when a part of the water tank 20 is taken as a curved surface, a condensing lens is formed by reservoir water stored in the water tank and the curved surface. When the condensing lens condenses sunlight, and radiate the same to a photo-electric energy conversion element represented by a solar battery 30, large electric energy can be obtained with a small photo detecting area. Thus, a temperature rise of a building where the water tank is set can be prevented by evaporation latent heat of the reservoir water, and the electric energy obtained by the conversion element can substitute for some of the energy consumed before so that sunlight can be used very efficiently.