To prevent process gas from leaking to the atmosphere by arranging a blow-by preventing fin which is projected outward of a radial direction from a sleeve disposed on a shaft side and for stopping flow of process gas between a process side labyrinth and an intermediate labyrinth, on an exhaust port for returning purge gas to a process side.

Since air supply pressure of a purge gas supplied from an air supply port 4 through an air supply port 12 is controlled so as to set slightly higher than an exhaust port 5, process gas is prevented from leaking to the atmosphere. When an abnormal situation is generated in this condition and process gas becomes high pressure, a blow-by phenomenon of process gas is generated, and areas of a process side labyrinth 7 and an intermediate labyrinth 8 are cut off by a blow-by preventing fin 10 arranged on a position of an exhaust port 5 and formed on a sleeve 2 attached to a rotary shaft 1, and the process side labyrinth 7 formed on a seal housing 6 side, and thereby, blow-by of process gas toward the atmosphere is prevented.