A first-information obtaining unit that obtains first information and a second-information obtaining unit obtains second information. The second information has a higher degree of importance than that of the first information. A generating unit generates watermark image data including a group of first dot patterns including the first information and a group of second dot patterns including the first information and the second information and having a dot size enlarged from a dot size of the first dot patterns. A superposing unit superposes the generated watermark image data on a target image data.