To provide a binder suitable for a binding unit for conducting a document binding work. Many 1/2 rings 13, 14 are arranged at a predetermined interval at both sides of the backbone of the binder, and the backbone 12 is coupled to the 1/2 rings 13, 14 of both upper and lower sides via a thin hinge. Since the backbone 12 is not formed in a split hinge structure different from a conventional binder,the backbone of the binder is grasped by a holding mechanism, the binder is opposed to the backs of loose-leaf sheets. Thus, when a binding unit for closing the 1/2 rings by the engaging mechanism tobe engaged is used, the backbone unit can be surely held until the binding is finished.