PURPOSE: To prevent the entrance of water into a vehicle chamber by controlling a blower fan in such a manner as to reduce the blown air quantity when negative pressure in an air box is a determined value or more in a subjected device for taking outside air through a cowl grill and the air box.
CONSTITUTION: A negative pressure sensor 21 detects the negative pressure in an air box 2 and input it to a control circuit 22. The control circuit 22 compares the detected negative pressure with a first set value, and, when it is a set value or more, the speed of a blower fan 12 is changed to 2nd speed, and a heater 20 is operated. Thereafter, the detected negative pressure of the negative pressure sensor 21 is compared with a second set value smaller than the first set value, and when it is lower then the second set value, the speed of the fan 12 is returned back, and the heater 20 is stopped. According to this constitution, an increase in negative pressure in the air box at the time of clogging a cowl grill can be prevented to drain water in the air box outside of the vehicle, and the entrance of water into the vehicle can be prevented.