For example, a conventional hole plug has, as shown in FIG. 20, an integral structure formed of synthetic resin and the like. In an insertion part 1, a front side flange part 2 and a back side flange part 3 are formed to face each other. The hole plug seals an attachment hole from both sides of the attachment hole by means of the front side flange part 2 and the back side flange part 3 (Patent Literature 1).