A single-fin fastener can be used in which an engaging fin has a flexible hinging butt at a tip of a support board projecting from a base member or the like and extends from the butt in a direction opposite that of the projection of the board. The fin includes an engaging notched portion and a stopper extending therefrom to a movable tip of the fin. An opening preventive portion is continuously attached to the base member or the like and faces the stopper across a gap.