A second connecting portion 22 of a second inside portion 2A is formed with an opening portion 24 extending all the way from an inner peripheral surface of the connecting hole 23 to an outer surface of the connecting portion 22 and towards a second outside portion 2B. This opening portion 24 is set both in size dimension and configuration such that a fitting portion 32a of a hinge pin 32 can be inserted into the connecting hole 23 via the opening portion 24. The second outside portion 2B is formed with a shielding portion 25 for shielding the opening portion 24. The shielding portion 25 is formed with a protruding portion 26 which is brought into the opening portion 24. A distal end face of the protruding portion 26 is generally in contact with the fitting portion 32a.