PURPOSE: To reduce abnormal noises caused by high frequency by a method wherein a gap is formed by making a hole in the volute wall of an air guide from the side of a motor of the air guide and thereby the exfoliation of air inside and around the volute wall after it runs against a volute tongue is reduced.

CONSTITUTION: The air pushed out to the outer periphery of an impeller 10 by the rotation of the impeller turns to be compressional waves divided in accordance with the number of sheets of blades 10a and runs against a volute tongue 8a, and after that, not flowing along a volute wall 8, exfoliates near the outside of the wall when its flow rate is high, while exfoliating near the inside thereof when its flow rare is low, thus generating strange noises of high frequency. However, by providing a gap 16 in a part of the volute wall 8, air is supplied from the inside of the wall 8 through the gap 16 at the time of high flow rate, thereby the exfoliation near the outside of the wall being reduced, while at the time of low flow rate the exfoliation near the inside of the wall being reduced, thus the occurence of the abnormal noises of high frequency being prevented.