PURPOSE: To make it possible to reliably prevent an air conditioner from being frozen even when there is an error in the connection of a refrigerant pipeline by detecting a refrigerant temperature on the outlet side of a decompression device in a refrigerating cycle within an outdoor unit and making a compressor stop its operation when the detected refrigerant temperature is lowered down to a predetermined temperature or below.

CONSTITUTION: In the case of the operation of an air conditioner, particularly in the case where either one of indoor units 2 and 3 carries out a cooling operation; when a refrigerant pipeline for the indoor unit 2 is erroneously connected to a solenoid opening and closing valve 35 and a refrigerant pipeline for the indoor unit 3 to a solenoid opening and closing valve 34, or when the wiring of the indoor unit 2 is erroneously connected to a photo-coupler 38 side and the wiring of the indoor unit 3 to a photo-coupler 37, refrigerant flows into one of the indoor units, which needs no cooling operation. Since this indoor unit is at rest and a blower is in the state of no operation, heat-exchanging is hardly effected by a heat exchanger, and a refrigerant temperature on the outlet side of. a decompression device 9 or 10 becomes lower. The lowering of this refrigerant temperature is detected by a temperature detector 36 to stop the operation of a compressor 4. By this method, the indoor heat exchanger can be prevented from being frozen.