To prevent fretting wear to be caused by axial elastic deformation of an outer disc due to thrust of a pressing device.

At least on one of one axial side face of a lock ring 13 and an axial outer side face of an outer disc 2b facing the one axial side face of the lock ring 13, an oil retaining recess is provided for retaining oil film of a lubricant. An O-ring 21 is held between one axial side face of a suppressing ring 14 which prevents the lock ring 13 from coming off from a lock recessed groove 12 and another axial side face of the outer disc 2b, thereby preventing the lubricant from flowing out in a radial direction.