PURPOSE: To produce hot water while sun light take-in parts are mounted not on a roof surface of a building but as a part of an interface wall on the ground by a method wherein the sun light take-in parts for collecting sun light at a heat collecting part are arranged on both flat surfaces of a heat collecting box.

CONSTITUTION: A heat collecting box 14 is provided with sun light take-in parts 50 for transmitting and heat collecting toward a heat collecting part 26 at both flat surfaces. The sun light take-in parts 50 are comprised of some transparent plates such as glass plates or transparent acrylic plates installed at both openings of the surfaces of the heat collecting box 14. Their peripheral edges are engaged with both surfaces of the heat collecting box 14 with a pressing frame. It is preferable that the heat collecting box 14 is arranged in such a manner that the sun light take-in parts 50 at both surfaces direct toward a north-south direction. Inner heat collecting parts 26 are heated with sun light radiated from the south side and a partial sun light is reflected by a reflector arranged at the north side and collected at the heat collecting parts 26. With such an arrangement as above, the heat collecting parts 26 are supported vertically by a supporting table 12 to reduced the rate of heat collecting of sun light, thereby both surfaces of the heat collecting box 14 are applied with sun light through the sun light take-in parts 50 and then a reduction of the rate of heat collecting operation is accommodated.