To prevent the abnormal abrasion of a seal ring.

Plural annular grooves 13 for pressure oil are formed on an inner peripheral face of an outer body 11 of a joint outer 10 with a rotatable joint inner 20 fitted therein, a back ring 15 and the seal ring 16 are concentrically fitted in annular grooves for seal formed at both sides of each annular groove 13 for pressure oil, and a semi-cylindrical fixed groove 17 is formed in the radial direction of the seal ring on an end face of the seal ring 16. The inner seal ring 16 and the outer back ring are fixed to each other by partial engagement of the back ring 15 with the fixed groove 17. Whereby the phenomenon that the seal ring is abnormally abraded in sliding the seal ring to the back ring, and pressing the seal ring to a side face of the annular groove for seal by the pressure in accompany with the sliding, can be prevented by fixing the seal ring and the back ring.