PURPOSE: To improve the heating effect with the controlling of indoor radiation capacity to a constant level by arranging a joint heater so as to circulate hot water obtained by solar heat to a heat exchanger of an indoor radiator while that from an auxiliary heat source to another heat exchanger.

CONSTITUTION: Solar energy absorbed with a heat collector 3 is stored into a heat storage tank 4 and circulated to a first heat exchanger 18. A hot water obtained from an auxiliary heat source equipment 10 is circulated to a second heat exchanger 19. The flow rate on a outlet side of the heat exchangers in a circulation circuit is controlled with a room temperature thermostat 20 on the suction side of a convection an 16, while the operation of a heater is controlled with opening/closing valves 21 and 22. In addition, the flow rate is controlled with a thermostat 23 for detecting the temperature on the up-stream of the first heat exchanger. When the system operates satisfactorily with the first heat exchanger, the second heat exchanger is halted but when it does not, the heat exchanger is used jointly. As a result, solar energy can be utilized to its utmost thereby ensuring an efficient heating capacity.