Abstract: A rotating member such as a sprocket to be coupled with a shaft is formed with a fitting bore which extends on the center axis and opens at at least one of its axial end portions. The rotating member is arranged in and rotatably held by a case such that its fitting bore opens toward the outside of the case. The shaft is inserted into and coupled with the fitting bore held in the case such that it can rotate integrally with the rotating member. This rotating member is formed with an air communication portion for providing communication between the inside and outside of the fitting bore and equipped with a valve for opening or closing the air communication portion. The valve may be opened in response to the rise or drop in the internal pressure of a fitting bore, by the pressure of the shaft or by a centrifugal force. With the valve being closed, lubrication oil in the case is prevented from leaking.