PURPOSE: To prevent deterioration of operationability of an impeller by making many holes at such positions as the air is blown out to the surface of a guide vane of a hollow shaft constructed rotatably to support a plurality of guidevanes thereby blowing the air to the hollow shaft and preventing adhesion of dust.

CONSTITUTION: Many radial holes 13 are formed in the direction to blow out the air or gas to a hollow shaft 6 constructed rotatably to support a plurality of guidevanes 7 along the surface of the guidevane 7. With such structure, when feeding the air or gas with predetermined pressure to the hollow shaft 6, it is blown out through many holes 13 made in the hollow shaft 6 to blow out dust adhered at the upper and lower end sections of the hollow shaft 6 while also blown out along the surface of the guidevane 7 to blow out dust adhered at the overlapped portion of the adjoining guidevanes 7 thereby deterioration of operationability of the guidevane 7 or stick due to adhesion of dust can be prevented.