Additional data useful for the management of copies of a document for an image forming apparatus is embedded in a hard copy of the document. The additional data is embedded in an inherent image of a document as pixels arranged in a prescribed format, and, preferably, the size of the pixels is such as to not be easily recognized with the naked eye. The image data for embedding the additional data can be obtained by reading a document, or by receiving data sent from a computer or read from a floppy disk. Additional data embedded in a hard copy can be extracted from the image data on the hard copy, and the production of a copy according to the image data can be controlled according to the extracted additional data. For example, if the additional data means that the source of the hard copy is a secret document, copying thereof is allowed only for a legitimate user. Additional data generated for each copy is compiled and is used to manage copies of various sources systematically.