PURPOSE: To prevent cabin temperature from going down during defrosting operation by adjusting the opening of inside/outside air switching door and decreasing outside air intake amount when an air intake mode is at outside air intake during defrosting operation.

CONSTITUTION: A controller 18 judges whether or not the heating operation is working from an air conditioning operation mode signal, and if so, judges whether or not the defrosting operation is working based on a defrosting operation signal by an deicer, then if so, detects an intake mode from an inside/ outside air intake mode signal. When the mode is the outside air intake mode, fresh mode, a control signal is outputted into the driving actuator of a switching door 4, and the switching door 4 is adjusted to the side of an outside air intake port 3 at approximately 1/3 opening to be set to the 1/3 fresh mode.