Disclosed is an air conditioning system for vehicles capable of operating control with a high degree of operating efficiency even in operating states wherein efficiency would be expected to decline, and while adhering to conventional control ideas. The air conditioning system for vehicles comprises a compressor that compresses a refrigerant; a condenser that condenses the compressed refrigerant; an expansion mechanism that expands the condensed refrigerant; an evaporator that evaporates the expanded refrigerant and brings the evaporated refrigerant into contact with air, cooling and dehumidifying the air; a blower that sends the air to the condenser; and a displacement control signal calculation means for determining a displacement control signal, using a displacement control signal calculation formula, which is to be inputted to the compressor from the outside. In the displacement control signal calculation means, a target air temperature at the evaporator exit is inputted, the displacement control signal is determined using the difference between the air temperature and the target air temperature at the evaporator exit when a displacement control signal calculated value is less than or equal to a predetermined value (A), and the displacement control signal is determined from the displacement control signal calculation formula when the displacement control signal calculated value is greater than the predetermined value (A).