To enhance heating capacity through hot gas bypath while protecting a compressor.

A correlation is present between the degree of superheat of delivery refrigerant gas of a compressor 10 and the quantity of cycle circulation refrigerant and heating capacity can be enhanced by controlling the degree of superheat of delivery refrigerant gas of the compressor within a specified range thereby bringing the suction refrigerant of the compressor into the liquid refrigerant returning state. A pressure sensor 26 and a temperature sensor 27 for delivery refrigerant gas are disposed on the delivery side of the compressor and detection signal from both sensors 26, 27 are inputted to a controller 29 in order to determine the degree of superheat. First and second solenoid valves 13, 20 are then turned on/off depending on the degree of superheat of delivery refrigerant gas from the compressor thus making a switching between discharge of refrigerant to the condenser 14 side and recovery of standing refrigerant from the condenser 14 side.