The image of a document is depicted on a sheet of paper together with a background pattern image expressing a password, and the password is notified to a person having authority to duplicate the document. A requestor requesting duplication of the document is caused to enter a password (#104), and the document image and the background pattern image are obtained by scanning the sheet (#101). Then, it is determined whether the requestor has the above-described authority, based on the entered password and the password expressed by the obtained background pattern image (#105). When it is determined that the requestor has the authority, a different password is issued. The document image is printed on a separate sheet of paper together with a background pattern image expressing the different password, and the different password is notified to a person having authority to duplicate the document printed on the separate sheet of paper.