PURPOSE: To prevent dust from entering the cabin from the outside even when window glass or a door is opened.

CONSTITUTION: When window glass is opened, a window glass switch 39 is opened, so that relay coils 35a, 36a are put in the current flowing state to close contacts 35b, 36b, and maximum voltage is applied to a fan motor 10. When a door is opened, a door switch 40 is opened, so that the relay coils 35a, 36a are put in the current flowing state to close the contacts 35b, 36b, and maximum voltage is applied to the fan motor 10. With the fan motor 10 thus driven by the opening of the window glass or door, outside air is led into the cabin, and the cabin internal pressure rises. Pressure difference is thereby generated between the inside and outside of a vehicle so as to prevent dust from entering the cabin.