<p>A planar section is formed on one or a plurality of ring sections of a binder, on an outer circumference side of a portion orthogonally intersecting with a spine section. When a binder holding member of a binding machine holds the binder, the planar section of the binder is permitted to abut to a planar table formed on the binder holding member. Since the planar section of the binder abuts to the planar table of the binder holding member when the binder holding member holds the spine section, the binder is held in a status where the binder cannot incline nor rotate and binding is performed by the binding machine with a prescribed posture. Thus, a possibility of causing a mounting failure due to inclination of the binder posture is eliminated.</p>
