PURPOSE: To obtain hot water in a constantly stabilized condition by a method wherein when the amount of insolation is sufficient enough, the hot water is obtained by the natural circulation of a coolant utilizing sunlight while when the amount of insolation is insufficient, heat generating during an adiabatic compression process by a compressor is utilized side by side with solar heat.

CONSTITUTION: In case where there is a sufficient amount of insolation, only a heat collecting plate 1 is used as a heat source. In other words, pipe passages A and B are connected by a three-way changeover valve 7, a solenoid valve 11 is closed so as to make a hot water tank 3 communicate directly with the lower part of the heat collecting plate 1 and a solenoid valve 5 is operated. As a consequence, high temperature steam within the heat collecting plate 1 is introduced into the hot water storage tank 3 through the solenoid valve 5 and the liquefied coolant is returned to the lower part of the heat collecting plate 1 through the three-way changeover valve 7. On the other hand, when the amount of insolation is insuffficient, the heat generated by the compressor 6 is used side by side with solar heat. In other words, pipe passages B and C are connected to each other through the three-way changeover valve 7 and the coolant is returned to the compressor 6 after it is gasified by an expan sion device 8 and sucked into the compressor through the hot water storage tank 3 and the heat collecting plate 1.