PURPOSE: To improve the calculation accuracy for the drive torque of a variable displacement compressor by utilizing the fact that the torque variation range at the time of operating the compressor in a maximum capacity region is generally small.

CONSTITUTION: The temperature of the outside atmosphere of a vehicle is detected by an outside air temperature sensor 100. The temperature in the vicinity of the outlet of cooling air current from an evaporator 40 is detected by an after-evaporation temperature sensor 110. The maximum capacity operation timing of a compressor 20 is judged by a microcomputer 120 based on the detected temperature after evaporation, while the drive torque of the compressor 20 is calculated corresponding to the detected outside air temperature based on the data indicating the relationship between the drive torque and the outside air temperature at the time of maximum capacity operation of the compressor 20.