A multi-gradation image such as a face image of a person in which sub-information is to be embedded is subjected to a preprocess by a preprocessing section based on a relationship between certain image data and read image data of a printed matter on which the certain image data is printed, sub-information is embedded into the multi-gradation image subjected to the preprocess by the preprocessing section by use of an digital watermark embedding section in an invisible state to the human eye, and the multi-gradation image having the sub-information embedded therein by the digital watermark embedding section is subjected to a post-process corresponding to the preprocess performed by the preprocessing section by use of a post-processing section.