PURPOSE: To reduce noise by making the inside of a hub part of a truncate come shape, which receives power from the outside and has a plural number of blades around it, a cavity structure, and forming a plural number of small holes which communicate with the cavity around the hub part.

CONSTITUTION: A plural number of blades 2 (4 blades are shown in the drawing) are mounted around a truncated one-shaped hub part 1 which receives power from the outside. The blade 2 has a front fringe part 3 and a rear fringe part 4, is tilted to the predetermined inclination, and rotates in the arrow direction 5. The inside of the hub part 1 constitutes a cavity structure, and a plural number of small holes 7 which communicate with a cavity 6 are formed around the hub 1. The cavity 6 and small holes 7 form a resonant type muffler. Noise due to air flow is reduced by this. Noise reduction effect is further improved by filling noise absorbing material into the cavity 6.