PURPOSE: To simplify a system itself and to enhance the overall performance coefficient particularly in the winter season by utilizing a solar heat collector as a dual purpose vaporizer in a heat pump cycle so that a solar heat collection system and a heat pump system may be made up integrally.

CONSTITUTION: A solar heat collector/vaporizer 1, a hot water tank 2, a condenser 3 in the tank 2 and a compressor 4 are provided. In this embodiment, the condenser 3 is placed in a position substantially higher than a thermal medium inlet header 11 of the vaporizer 1, and its one end is connected to a thermal medium outlet header 7 of the vaporizer 1 through a piping containing a solenoid valve 5, while the other end is connected to the header 11 through a piping containing a solenoid valve 8. The compressor 4 is connected inbetween the header 7 and one end of the condenser 3, while inbetween the other end of the condenser 3 and the header 11, a by-pass circuit 13 is provided which has an automatically expandable valve 12 that is parallelly connected relative to the solenoid valve 8. As a result, the year-round energy efficiency can be markedly improved.