PURPOSE: To miniaturize an auxiliary heater, economize fuel cost and the like by operating the auxiliary heater only when heat collecting operation stops and a hot water detector indicates that the water temperature is under a specified value.

CONSTITUTION: A solar heat collector 1 is connected with a heat exchanger 5 in a hot water tank 4 by a distributing pipe 2 through a circulation pump 3. Through a circuit sluice valve 6 on the way of the distributing pipe 2 is provided a bypass circuit which does not pass the heat collector 1 and an auxiliary heater 7 is connected on the way. At the bottom part of the hot water tank 4 is prepared a supply water pipe 8 and on the highest is a supply hot water pipe 9. Heat collection control is carried out by a temperature difference controller 13 where is used the difference between the temperature of the 1st thermometer 11 at the outlet of the heat collector 1 and that of the 2nd thermometer 12 at the bottom of the hot water tank 4. Only when the temperature of the hot water is low as it is cloudy the auxiliary heater 7 is used by day and the solar heat should be used preferentially.