PURPOSE: To enhance defrosting performance significantly by performing on/off control of a first control valve based on a temperature detected by a temperature detecting means and controlling the delivery temperature of refrigerant from a compressor thereby protecting a refrigerant compressor, a refrigerant hose, and the like, against damage.

CONSTITUTION: When the r.p.m. of an engine or the outer air temperature increases, the delivery temperature of refrigerant from a refrigerant compressor exceeds a second set level. A second temperature sensor 22 opens a second solenoid valve 32 by delivering a second on/off control signal thereto. Low temperature gaseous refrigerant not absorbing heat in an inner heat exchanger 6 is thereby fed through a four-way valve 4 to the inlet of the compressor 3 thus lowering the delivery temperature of refrigerant therefrom. During defrost cycle operation, a first sensor 21 opens a first solenoid valve 31 by delivering a first on/off control signal thereto. Low temperature gaseous refrigerant not absorbing heat in an outer heat exchanger 5 is thereby fed through the four-way valve 4 to the inlet of the compressor 3 thus lowering the delivery temperature of refrigerant therefrom.