PURPOSE: To decrease the variation range of temperature of discharged air at the outlet port of an evaporator by changing a constant for controlling the capacity of a compressor so as to increase and decrease the same in response to the temperature difference between the present measured value of the discharged air or the like and the value measured a predetermined time before, which ascends and descends around a set value as a reference value.

CONSTITUTION: A sensor 22 for detecting the temperature of coolant gas discharged from a compressor 2 rotated by an engine 1 is disposed in a pipe line 5 communicating between a discharge chamber 2 in the compressor 2 and a condenser 6. Meanwhile, in a computing and control device 17, there is provided a means for setting a desired temperature of blown-off air at the outlet port of an evaporator 8. Further, there is provided a means 24 for computing a difference between the present discharge temperature and the discharge temperature a predetermined time before, in accordance with a detection signal from the sensor 22. Further, there is provided a means 25 for changing a capacity control constant in accordance with the discharged air temperature difference, and computing a control value in accordance with thus changed value so as to deliver a control signal to a capacity changing mechanism in the compressor 2. Further, the capacity control constant is changed so as to be increased and decreased in response to the rise and fall of the discharged air temperature difference.