A fan motor is mounted on the upper wall of the casing. The lower surface of this upper wall is formed into an inclined surface in which inclination is formed from the front wall side of the casing toward a back wall across from the front wall. When the fan motor is driven, oil mist or moisture contained in the air are condensed into water droplets or oil droplets. These water droplets or oil droplets fall and flow to the inner wall surface of the upper wall of the casing along the inner peripheral wall of a vent hole. Then the water droplets or oil droplets flow along the profile line of a vent hole smoothly (without stopping) and roll down the side wall of the casing.