PURPOSE: To keep low the condensation pressure of a heat pump and improve its energy efficiency by connecting one party of cylinder for a compressor, where a plurality of cylinders are built inside a case, with a high temperature radiator and connecting the other party of cylinder with a low temperature radiator.

CONSTITUTION: A compressor 30 is provided with two cylinders 31 and 32. A pipeline 5a extended from a vaporizer 4 is open in a case 33. A high temperature radiator 35 and a low temperature radiator 36 are laid out top and down with a span in a hot water supply tank 34. The high temperature radiator 35, located in the upper part, is connected with the cylinder 31 by way of a pipeline 5b on one end and connected with the vaporizer 4 on the other by way of a pipeline 5d end; while the low temperature radiator 36, located in the lower part, is connected with the cylinder 32 by way of a pipeline 5c on one end and connected with the radiator 4 by way of a pipeline 5e on the other end. Refrigerant control valves 37 and 38 are laid out in the pipelines 5d and 5e respectively. On the other hand, a hot water supply tank 34 is connected with a feed pipe 39 in an elevation lower than the low temperature radiator 3 and connected with a water outlet pipe 40 in an elevation higher than the high temperature radiator 35.