PURPOSE: To make it possible to improve the heat collecting efficiency and holding the temperature of supply hot water at a predetermined value by warming city water in a reserve tank by a solar heat collector, and shifting warmed water to a hot water storage tank to store warmed water therein.

CONSTITUTION: A heat medium warmed in a solar heat collector 10 circulates within a circulation path 16 and is heat-exchanged with city water in a reserve tank 12. When the temperature of water within the reserve tank 12 is raised to a predetermined value, valves 28 and 30 are opened to supply water into the reserve tank 12. Hot water existing in an upper layer part within a reserve tank 12 is pushed up by city water in a water feed pipe 14 and moves into a hot water storage tank 18 via a connecting pipe 24. The reserve tank 12 is constantly filled with city water, and the difference in the temperature between the solar heat collector 10 and the temperature of water within the reserve tank 12 is large, and hence the heat collecting efficiency and the heat collecting amount are improved. Further, the temperature of water within the water storage tank 18 becomes constant regardless of the difference in the amount of solar radiation, and the temperature of hot water supplied through a hot water supply pipe 26 can be maintained at a predetermined value.