PURPOSE: To improve air blasting to the upper half of a rider's body at the time of low outside air temperature so as to improve feeling and the like by determining blow-off modes by the sequential pattern of a heat mode, a differential heat mode and a bi-level mode at the time of low outside air temperature.

CONSTITUTION: Air blow-off modes are switched by plural mode doors 22a, 22b. Heat load is detected by a means 100, and a mode switching parameter is computed by a means 200 on the basis of the heat load. According to the largeness of the mode switching parameter, means 300, 400 assign plural modes out of a specified pattern provided with specified mode permutation. Whether outside air temperature is lower than the specified value is judged by a means 500, and the above-mentioned plural modes are selected by a means 600 according to the judged result. On the basis of the respective selected modes, the driving of the mode doors 22a, 22b is controlled by a means 700.