To prevent shape of a guide member from being complicated by providing a reinforcing ring on the outer peripheral side of a resin sliding ring, backing a backup part of a rubber elastic material backing the sliding ring up on the inner peripheral side of the reinforcing ring and backing a check lip on the outer peripheral side of the reinforcing ring.

When pressure within a hydraulic chamber 35 is increased, a packing 1 primarily seals oil O and an oil seal 27 seals oil which passes between a sliding ring 2 and a rod 24 and enters space 38. Each of the backup part 7 and the seal projection 9 of the packing 1 is brought very close to an end face 23a of a guide member 23 and prevents oil from being leaked to the outer peripheral surface. The sliding ring 2 is backed up by the backup part 7, is abutted on the flange part 5 of the reinforcing ring 3 and maintains its location and attitude, although the sliding ring 2 is pressed by pressure of the hydraulic chamber 35 and is going to move to a low pressure side. When oil is accumulated in space 38 and pressure accumulating phenomenon is generated, pressure acts on a check lip 8. When pressure becomes prescribed pressure, a reserve chamber 36 is forcibly opened and oil is collected to the receive chamber 36.