Abstract: A cap body is formed with a support sleeve and an arcuate guide projection wall which are concentric with a mounting base plate. The support sleeve is formed with a first notch and a first guide groove. A first engage claw is projected from a pivot shaft of a shield plate and enters and comes out of the first notch. The first guide groove engages with the first engage claw through the first notch. A lock lever is pivotally supported by the mounting base plate, and is provided with a first and second lock claws which engage and disengage with first and second engage claws in accordance with a locking position and an unlocking position of the lock lever. Therefore, at the attaching and detaching position of the shield plate, the shield can be locked at least at two points by a single lock member and thus, a coupled strength of the shield plate is increased.