A refrigerating system, using CO.sub.2 as a refrigerant, for an air conditioner for an automobile. A pressure at the outlet of the heat emitter 2 is controlled to a target value in accordance with the temperature of the refrigerant at the outlet of the heat emitter for obtaining an operation of the refrigerating system along the optimum control line .eta..sub.max. A pressure reducer 4 and a cooler 5 are provided on a branched passageway 28 branched from a refrigerant recirculating passageway 27 for injection. The cooler 5 is for obtaining a heat exchange between a flow of the refrigerant on the branched passageway 28 after being cooled by the pressure reducer and a flow of the refrigerant on the main passageway 27, thereby cooling the refrigerant directed to the evaporator. In a second aspect of the invention, a means is provided for increasing a target value of the pressure at the pressure reducer when a thermal load of the refrigerating system is high. In a third aspect, a series of pressure reducers are arranged on the main recirculating passageway. The second stage pressure reducer is controlled so as to obtain a desired value of degree of super heating at the inlet of the compressor.