PURPOSE: To improve sealing performance, by providing an O ring in a grooved part which is formed by a reverse L-shaped groove, provided in a plate ring in its atmospheric side, and by a ringshaped disc, having an internal diameter larger than the small contour part of the reverse L-shaped groove, in its atmospheric side.

CONSTITUTION: An O ring 5, which is placed being put between a plate ring 2 and a ring-shaped disc 15, is always compressed by a pressure of sealed fluid. Since the compressed force of the O ring 5 acts in directions of its contact surface with a housing 4 and a cylindrical surface of the plate ring 2, the pressure of these contact surfaces is generated in proportion to the pressure of sealed fluid, and since the more is improved the sealing action the higher the pressure of the sealed fluid increases, the O ring 5, even if its rubber material elastic force weakens, is prevented from deteriorating the sealing force.