PURPOSE: To enable comfortable air conditioning, by using an actuator to control the degree of opening of an internal and external air changeover damper depending on the outputs of sensors which detect the relative humidity of the room of an automobile and the dew formation of the window pane, to regulate the relative humidity.

CONSTITUTION: When air conditioning is started, the outputs of sensors 9, 10 which detect the humidity of the room of an automobile and the fogging of the window pane and the data on the positions of an internal and external air changeover damper 7 and a mode setting means are entered into a microcomputer 11. The degree of opening of the damper 7 is calculated on condition that the window pane is not fogged, so that the damper is controlled to an internal air position if the humidity of the vehicle room is not higher than 20% and that the damper is controlled to an external air position if the humidity is 50%. An actuator 8 is driven depending on the result of the calculation to shift the damper 7. When the window pane is fogged, the damper 7 is shifted to the external air position.