When an assembly in which a sprag is temporarily assembled to a retainer by a return spring is assembled between an outer race and an inner race, even if the sprag projecting from the retainer is depressed, a drop-out of the sprag is prevented by a retaining member of a friction plate fixed to the end portion of the retainer. A lubrication passage is formed by opening a space surrounded by the outer race, retainer, and sprag to the outer peripheral side. An attaching member of the friction plate which is fixed to the retainer by a spot welding is recessed backward by joggling in a downward stairway manner for a contact surface. Therefore, even if a rise occurs in the attaching member by the welding, the rise is not come into contact with the contact surface. A corner edge is formed in a spherical shape in a manner such that when the corner edge of each of the frictional members divisionally formed on the outer periphery of the friction plate is dropped into a rivet hole of the attaching surface and, when it comes out from the rivet hole, the corner edge does not thrust into the periphery of the hole.