An upper cover has an engagement edge, a bottom cover has a receiving edge where the engagement edge is inserted, and the engagement edge is inserted into the receiving edge. The receiving edge has a groove where the leading end of the engagement edge is inserted, the engagement edge has a stopper surface, which serves as a stopper part, on the outer surface toward the basal end side, and the groove has a stopper receiving surface, which serves as a stopper receiving part and catches the stopper surface, on one side internal surface toward the basal end side. As the leading end of the engagement edge is inserted into the groove of the receiving edge and the stopper surface is caught by the stopper receiving surface, the engagement edge of the upper cover and the receiving edge of the bottom cover are joined together without any separation.