To reduce noise without decreasing blast effect.

A plurality of double level surfaces 8a, 8b are formed in a cover 8 via a step 8c along a stream direction of air, and a plurality of bleeders 9a, 9b are formed at the plurality of double level surfaces 8a, 8b, respectively. By this constitution, the bleeders 9a, 9b are arranged at positions different from each other with respect to a moving blade 7. Accordingly, the frequency wavelength of the noise generated when the air passes the bleeders 9a, 9b can be shifted by a half of the frequency.