An opening portion 111 includes a plurality of extended portions 111a that are each a portion of the opening portion 111 with an enlarged diameter and a notch 111b formed in an edge portion. An insulator 6 includes a support 6a that has a larger diameter than that of the opening portion 111, a trunk portion 6b that has a smaller diameter than that of the support 6a and that can be inserted through the opening portion 111, a plurality of lock portions 6c, the diameter of which is partially enlarged from the trunk portion 6b, that can be passed through the extended portions 111a, a hook portion 6d that is formed on a lateral side of the trunk portion 6b and that can be engaged with the notch 111b, and engagement holes 6e that are formed in a surface that is in contact with the stopper 5, the engagement holes 6e engaging thereto a jig 8 for fixing the insulator 6 to the opening portions 111.