An air conditioning system for a vehicle has a refrigerant cycle device which has an evaporator and a compressor and in which refrigerant is discharged from the compressor to be evaporated in the evaporator so that air passing the evaporator is cooled to be blown into a passenger compartment of the vehicle. The control unit drives the compressor at a control value which is determined based on at least one of a pressure of the refrigerant in the refrigerant cycle device and an outside-air temperature outside the passenger compartment, when the control unit actuates the compressor.