To ensure satisfactory heating without the need of a heat source auxiliary apparatus even in the severe cold time by constructing a gas compressor into 2 stages, and cooling discharge gas from a low pressure side gas compressor with a cooling apparatus and feeding it to a high pressure side gas compressor and simultaneously controlling the gas compressor in response to atmospheric temperature.

A gas compressor consists of a high pressure side gas compressor 1 and a low pressure side gas compressor 2, and cooling and heating are changed over by a four-way valve 3. There are further provided three-way valves 4, 5 controlled in response to outdoor atmospheric temperature each of which serves to change over whether both two stages of the gas compressors are operated or whether only any one of the low pressure side gas compressor 2 and the high pressure side gas compressor 1 is operated. An indoor heat exchanger 6 is provided on a refrigerant circuit which serves as an evaporator at the time of cooling to lower indoor temperature with heat absorption while dissipating heat as a condenser at the time of heating. Further, an outdoor heat exchanger 7 is provided which serves as a condenser at the time of cooling while serving as an evaporator at the time of heating to absorb heat from the outdoor atmosphere.