To quicken the response corresponding to gas low condition and protect a compressor without fail, by stopping the compressor through an electric contact which closes when the piston shifting according to the pressure difference between the pressure of the suction gas refrigerant sucked in the compressor of a freezer and atmospheric pressure shifts by a specified quantity.

In operation of an air conditioner for a vehicle, when the load of the air conditioner is small, if the pressure within a pressure chamber 25 leading to a suction chamber 13 drops, a piston 21 shifts right and left, according to the pressure difference between the pressure of a suction gas refrigerant and the atmospheric pressure. Then, a suction gas refrigerant passage 10 is throttled by the cut 29, and the quantity of a gas refrigerant sucked in the compressor becomes small, and the capacity of the compressor 1 drops. When it comes to gas low condition by the leakage of the refrigerant charged in a refrigerant circuit, the pressure of the suction gas refrigerant drops more, and the piston 21 shifts right more, and the electric contact 31 contacts with an electric contact 32, whereupon the electric circuit leading to an electromagnetic clutch becomes ON, and the electromagnetic clutch becomes OFF, and the compressor stops.