A pipe includes a cutout portion through which levers of a ring gear enter the pipe. An inner peripheral surface of the pipe opposing the cutout portion includes a curved inner peripheral surface in a curved portion of the pipe, a linear inner peripheral surface in a linear portion of the pipe, and a transition portion from the curved inner peripheral surface to the linear inner peripheral surface. The transition portion is located closer to a position where balls start to press the levers of the ring gear, than an intersection where an imaginary straight line α extending from a cutout end portion of the cutout portion intersects an imaginary extension line of the inner peripheral surface of the linear portion of the pipe at a right angle.