An air conditioning system for a vehicle has a compressor adapted to vary its capacity so as to maintain the suction pressure of refrigerant. A door is arranged upstream of the evaporator in an air passage, which can assume a variable opening to vary the refrigerating capacity of the evaporator. The opening of the door is controlled in response to the set position of temperature setting means which sets the discharge air temperature, whereby the refrigerating capacity of the evaporator can be reduced when overcooling takes place, to thereby reduce the work done by the compressor for minimizing a waste of energy.