PURPOSE: To reduce the clearance between a rotary shaft and an air jet member, even in the case where a temperature rises by rotation of the rotary shaft at a high speed.

CONSTITUTION: When a rotary shaft 36 is rotated at a high speed to generate heat, a temperature rises in the rotary shaft 36, bearing bases 17, 18 and air ejecting member 31, 32 to perform thermal expansion, but since the air ejecting members 31, 32 with a coefficient of thermal expansion smaller than the rotary shaft 36 is fixedly inserted to the bearing bases 17, 18 with a coefficient of thermal expansion larger than the rotary shaft 36, the air ejecting members 31, 32 are largely expanded by receiving an influence of the bearing bases 17, 18, to make expansion (enlarging internal diameter) of the air ejecting members 31, 32 approach expansion (enlarging external diameter) of the rotary shaft 36.