A number of ½ ring parts (13 and 14) are arranged at constant intervals on both sides of the spine portion (12) of binders, and the spine portion (12) and the ½ ring parts (13 and 14) on both upper and lower sides are coupled together via thin hinges. As the spine portion (12) is not in double-split hinge structure, which is different from the case of conventional binders, it is ensured that the spine portion is held until the termination of the binding operation when a binding apparatus is used for causing a holding mechanism to clutch the spine portion of the binders so as to set the binders to face the back of sheets of loose-leaf paper and also causing an engaging mechanism to close and engage the ½ ring parts.