PURPOSE: To efficiently utilize natural energy by placing heat storage material above or near the surface of a pool of water in the daytime and by submerging it under the water in the nighttime.

CONSTITUTION: For the daytime, air is sent into floating bodies 5 through a conduit 7 by an air pump 6; a casing 4 filled with a heat storage material 3 is thereby made to rise above the surface of a pool of water through displacement of the water from the floating bodies 5 through vents opened therein. The heat storage material 3 in the casing 4, which has surfaced, is heated by solar radiation and stores the heat both as sensible heat and latent heat. For the nighttime, the air is discharged from the bodies 5 so that the casing 4 thereby sinks in the pool of water through influx of water into the floating bodies 5 through the vents. During the submergence, the heat stored in the heat storage material 3 in the daytime is gradually released to the surrounding water to heat it. The heated water rises in a convective flow through decrease of the specific gravity and the temperature of the water is uniformized in the pool through mixing of the water between the upper portion and the lower portion thereof. Natural energy can thus be efficiently utilized in a simple manner and at low cost and uniformity in the water temperature can be achieved in the application to a breeding pond, reservoir, and the like.