In the embodiment, a gas such as the air, which is supplied from the exterior, flows into a gap between the outer pipe 14 and the rotary shaft 15 via the inflow opening 26 shown in FIG. 2, further flows into the gas flow passage 18 between the rotary shaft 15 and the inner pipe 16 via a gas inflow opening 36 which is formed in a lower portion of the rotary shaft 15 as shown in FIG. 3, and is then