Additional information is embedded in document data by inputting document data, inputting additional information, and inputting image data. The additional information is embedded in the input image data using a first digital watermark method, and the additional information is embedded in the input document data using a second digital watermark method. The document data embedded with the additional information is composited with the image data embedded with the additional information, with the image data as a background image. In a case where an original document containing information embedded using the second digital watermark method is copied by a copying machine, the information embedded in the copied document cannot be detected. The first digital watermark method has a robustness such that, when an original document containing information embedded using the first digital watermark method is copied by a copying machine, information embedded in the copied document can be detected.