A backlash comprises a first rotation member and a second rotation member, each of which is provided rotatably-about the same axis, and which are engageable together in a rotating direction of the members by driving contact between respective radial extension surfaces thereof,and where a backlash gap is provided between the respective radial extension surfaces of the first and the second rotation members to permit fee rotation between the first and second rotation members in a range of the backlash gap in the rotating direction of the members.