The waste heat from a heating part mounted on a vehicle is recovered and extracted by a refrigerant for gas injection so that the heating capacity at a low ambient temperature may be effectively improved while suppressing an increase in the power consumption. At the heating time, the refrigerant is circulated in a closed circuit composed of a compressor, an indoor heat exchanger, a pressure regulator and an outdoor heat exchanger, and the refrigerant, as condensed in the indoor heat exchanger, is regulated to an intermediate pressure by a pressure regulator and introduced into a heat exchanger. In this heat exchanger, the refrigerant extracts the heat from the heating part at the side of the vehicle so that it evaporates. The refrigerant thus having evaporated is introduced through an evaporation pressure regulating valve via a gas injection passage into a gas injection port of the compressor.