To provide a binder into/from which documents can be inserted/removed easily and in which the documents can be easily sorted, for solving the problem of troublesome work that, in the case wherein the volume of documents in a conventional file binder for binding the documents having filing hole is increased and so the documents are added to or removed from the conventional file binder or are sorted in the file, the documents bound in the file binder by a retainer (binding string, band, pin, etc.) once have to be removed from the file binder and a large volume of the documents have to be bound again.

In order to provide the filing system for binding a small file (unit file) freely detachably into a master file (file binder), the small file is provided with a fitting gap corresponding to the retainer of the master file and a grip for opening/closing the gap so that the small file can be removably bound in the master file.