Abstract: One end of a seal ring retained in a seal ring groove is formed in such a manner that it is offset radially inwardly from the other end, so that oil is permitted to leak by opening an abutment joint formed by the two ends in a condition in which no hydraulic pressure is applied. If a hydraulic pressure is supplied to bring a hydraulic actuator into an engaged state, the one end of the seal ring is deformed radially outwardly by the hydraulic pressure to come into close contact with a sealing surface and to close the abutment joint, thereby preventing the oil from-leaking past the oil ring. Consequently, upon engagement of the hydraulic actuator, a sealing property or salability of the oil can be assured, and upon releasing of the engagement of the hydraulic actuator, an oil leaking property of the seal ring can be provided, leaking property of the oil can be provided, thereby enhancing the responsiveness of the hydraulic actuator.