To prevent the loss of a warm feeling at a foot part as much as possible by diffusing air from both of a defroster diffusing port and a foot diffusing port even when a defrosting command is issued.

While a defroster switch 52 is ON, a diffusing port mode immediately before the turning ON of the defroster switch 52 is determined. When the mode immediately before the switch turning ON is one other than a vent mode, a heat quantity Q1 necessary for window defogging is compared with an actual heat quantity Q2 obtained by current air conditioning control. If Q1<Q2 is determined, a superfluous heat quantity Q3 is calculated. Then, based on Q3, the opening of a defroster door 21 is calculated. Then, the defroster door 21 is driven by a mode actuator 34 so as to reach the calculated opening.