PURPOSE: To reduce the noise level by disposing a shutter for preventing outside air from entering on the delivery side of a centrifugal fan and an ellipse- sectioned air guider at the leading edge of the shutter.

CONSTITUTION: When a centrifugal fan 5 is operated, the room air is vented to the outside through outlet 6, interior of adapter 7 and duct 8 as shown by arrows. The shutter 10 is forced open by the air pressure. Because an elipse- sectioned air guider 11 is disposed at the leading edge of the shutter 10, the draft resistance of the delivered air is reduced, and the air is delivered through the periphery of the shutter 10 and the interior of the adapter 7 without causing swirls. By this constitution, the noise level can be reduced.