PURPOSE: To permit to heat water in a heat accumulating tank, whose volume is proportional to the receiving amount of solar enerby, to a constant temperatue automatically by locating the heat dissipating section of a heat pipe in the heat accumulating tank.

CONSTITUTION: The heat dissipating sections 1a of a plularity of heat pipes, whose heat collecting sections 1 are located at a heat collector, are located in the heat accumulating tank 2 through packings 7. An opening and closing valve 3 is connected to the tip end 1b of one heat pipe among a plurality of them. Water-tightness of the heat accumulating tank 2 and the tip end 1b is kept by a packing 8. Feed water, passing through the opening and closing valve 3 connected to a feed water pipe 4, is discharged into the heat accumulating tank 2 through the opening 5 of the feed water pipe and is supplied to the bottom of the tank 2 through the feed water pipe 6. The heat collecting section 1 of the heat pipe receives the solar beams and the temperature of refrigerant therein is risen. When the temperature of the refrigerant arrives at a value, the opening and closing valve 3 is opened by a pressure rise, water feeding is begun, and the heat dissipating section 1a is cooled by the feed water. When the temperature of the heat dissipating section 1a arrives at a value by the cooling of the feed water, the valve 3 is closed, the heat of the heat collecting section 1 moves sequentially to the heat dissipatong section 1a to heat the feed water. When the temperature of the feed water arrives at 60°C, water feeding is restarted and heated water is pushed up by the feed water, therefore, the heat dissipating section 1a is cooled again and water feeding is stopped when the temperature of the section 1a has arrived at 30°C. Heated water having a temperature near the set temperature 60°C is reserved in the tank 2 by repeating above-described cycles.