Abstract: A bottom portion 11a is formed on an external end portion of a support sleeve portion 11 which is located on an external side, and an engagement recess 43 is formed in an inner peripheral surface of a support sleeve portion 21 which is located on an internal side. That end face of the engagement recess 43 which faces the support sleeve portion 11 side serves as an engagement surface 44. An engagement piece 45 is formed on one end portion of a hinge body 3. The engagement piece 45 is elastically deformed such that its distal end portion is displaced radially inward of the hinge body 3, thereby the engagement piece 45 is allowed to extend beyond a bottom surface of a key groove 41. Thereafter, the engagement body 45 is elastically restored such that its distal end portion is displaced radially outward of the hinge body 3, thereby the engagement piece 45 is engaged with the engagement recess 43.