PURPOSE: To fractionalize and store the thermal energy or coldness in different temperature fractions by providing a window near the top of the frame or side of a fractionalizing box which has a pipe projecting from its floor and a feed pipe from a heat collector whose end is inserted below the window.

CONSTITUTION: One or multiple windows 3 are provided in the top or upper part of the side of the frame 2 of a fractionalizing box submerged inside the outer casing 1 of a heat accumulation tank. A pipe 4 of a highly heat conductive material protrude downward from the floor, and its lower end 6 is open to the inside of the outer casing 1 of the heat accumulation tank. The end 8 of a feed pipe 7 from a heat collector 9 is inserted to a suitable position below the window 3. By supplying tap water or ground water at a certain pressure from a water supply port 12, the inside of the outer casing 1 as well as the frame 2 of the fractionalizing box is filled up. The heat collector 9 rises in temperature by being exposed to the solar heat, while the heat accumulation tank accumulates the heat with temperature- fractionalized layers of H range = high-temperature range, M range = medium temperature range and L range = low-temperature range with the window 3 and the lower end 6 of the pipe 4 as the boundaries. Since the high-temperature water in the heat accumulation tank is taken out for usage from an outlet 13 by the pressurized tap water etc. from the water supply port 12, the temperature-fractionalized layers are not disturbed.