On an inner peripheral surface of an outer peripheral member 10, a recess 11 is formed to which a pipe spring 30 is fitted. On an outer peripheral surface of an inner peripheral member 20 arranged concentrically with and rotatable relatively to the outer peripheral member 10 on an inner peripheral side of the outer peripheral member 10, a cam surface 21 is formed which abuts against the pipe spring 30. The cam surface 21 protrudes in the radial direction as it is farther away in the circumferential direction of the outer peripheral member 10 from the point of contact with the pipe spring 30 in the state where no load is applied.