PURPOSE: To enable to efficiently separate and collect the water with the temperature above the predetermined one by utilizing the difference of specific gravity by a structure wherein a horizontal floating plate, the specific gravity of which is larger than that of the hot water to be collected and at the same time smaller than that of the cold water to be supplied in the system of the device, is arranged in the passage for circulating liquid.

CONSTITUTION: A hot water collecting unit 1 consists of the horizontal floating plate 4 installed within a cylindrical vessel 3. The horizontal floating plate 4 floats at the boundary surface of the two liquids, one of which is the cold water lying below a plate 4 and the other of which is the hot water lying above the plate 4 by employing the specific gravity of the plate 4, which lies within the range of specific gravity smaller than that of the cold water lying below the plate 4 and as well as larger than that of the hot water lying above the plate 4. Furthermore, by regulating the specific gravity of the horizontal floating plate 4 so as to be equal to the specific gravity of the water with desired temperature, the hot water with desired temperature is stored at all times above the horizontal floating plate 4 through a piping 18 regardless of the fluctuation of the heat quantity received at a solar heat collector 2. In addition, when hot water is stored so much above the plate 4 as to let the plate 4 down till to attain to the bottom of the vessel 3, valves 13 and 15 are open and at the same time a valve 16 is closed, resulting in sending the hot water stored above the plate 4 into a heat accumulator 12.