PURPOSE: To reduce noises without deteriorating basic performance by making a hub part have a hollow structure, forming a small hole which penetrates from the surface of the hub part to a hollow part, or, forming a bellmouth in a hollow structure, and providing a small hole which penetrates from the surface thereof to a hollow part.

CONSTITUTION: A bellmouth 6 having a hollow structure, has a small hole 8 penetrating from the surface there to a hollow part 7, and a sound absorption material 9 is filled in the follow part 7. The hollow part 7 functions as a resonance muffler which reduces a noise caused by air flow or the like, and the noise is further reduced by charging the sound absorption material 9 in the hollow part 7. The above-described method does not affect basic performance, so that it is not necessary to change the performance design. The same effect is obtained by forming a hub part 11 in a hollow structure, providing a small hole 17 penetrating from the surface thereof to a hollow part 16, and charging a sound absorption material 18 in the hollow part 16.