PURPOSE: To prevent occurrence of vibration which is apt to occur in low speed rotation by connecting plural positions in circumferential direction of an annular atmospheric layer to each other outside of the annular atmosphere.

CONSTITUTION: Holes 12aW12c that connect an annular opening 10 formed between a cylindrical body 9 placed concentrically to a rotating body 3 and an annular opening 11 formed between outer face of the cylindrical body 9 and side wall 7 of the container are provided. Consequently, even when pressure in the opening 10 becomes locally high by deviation of axis center of the rotating body 3, the pressure is released to the part of low pressure atmosphere, and occurrence of unstable vibration can be suppressed effectively.