Four tapered portions are formed on an end portion of an inner wall surface of an air channel at four locations corresponding to four corners of the profile of a surface of a housing where a suction port is formed. The four tapered portions are each inclined outwardly in a radial direction of a rotary shaft from a discharge port side toward the suction port side and extending in a rotational direction of an impeller. The tapered portions each include a main portion which is shaped such that an angle formed between the main portion and an axis of the rotary shaft gradually becomes smaller from one end of the main portion located rearward as viewed in the rotational direction of the impeller toward the other end of the main portion located forward as viewed in the rotational direction of the impeller.