PURPOSE: To reduce noise by providing vanes at uneven spacings on the periphery of a hub in appro. cylindrical form, wherein the vanes shall have different projected figures on a plane which is perpendicular to the rotary shaft.

CONSTITUTION: Vanes 12W16 having different projected figures on a plane, which is perpendicular to the rotary shaft, a provided on a hub 11 in approx. cylindrical form. They are arranged on the periphery of the hub in such a way that they are not overlapped with one another, and the hub 11 shall be provided with notches 17 where the vanes 12W16 are overlapped with the hub 11. Thus the vanes are arranged around the hub at uneven pitches to suppress Nz sound, and at the same time, sufficient vane area is obtained, which ensures high performance.