PURPOSE: To prevent leakage of high-pressure air by a method wherein such a gap that a casing and a ring substantially do not contact with each other is provided between the casing and the ring by using a ring made of a hard, light-weighted resin.

CONSTITUTION: An annular groove 22 opening toward the casing 4 is provided at the lateral part of a vane wheel 21 which is faced to the casing 4 with a small gap therebetween. The ring 23 made of the hard, light-weighted resin such as polystyrene foam is fixed in the groove 22 by fitting under pressure. The pressure contact surface of the ring 23 is worn in a short period of time accompanying with the rotation of the wheel 21, and the gap substantially preventing the casing 4 and the ring 23 from mutual contact is formed between the casing 4 and the ring 23 after the blower is operated for a certain period of time. Accordingly, leakage of air at the part in question can be prevented.