1. An information embedding apparatus comprising: an image obtaining unit to obtain image information; a first-information obtaining unit to obtain first information to be embedded in the image information obtained by the image obtaining unit; a second-information obtaining unit to obtain second information to be embedded in the image information obtained by the image obtaining unit, the second information having a higher degree of importance than that of the first information; a generating unit to generate watermark image information including a group of first image patterns and a group of second image patterns, each of which is arranged at each of a plurality of embedding areas, wherein each of the second image patterns is obtained by replacing dots corresponding to one of the first image patterns with dots having an enlarged size, the first image patterns and the second image patterns respectively represent one bit or a plurality of bits that constitute(s) the first information, and sets of the second image patterns respectively represent one bit or a plurality of bits that constitute(s) the second information in such a manner that a positional relationship between the second image patterns in each of the sets represents one bit or a plurality of bits; and a superposing unit to superpose the watermark image information on the image information obtained by the image obtaining unit thereby obtaining watermark-superposed image information, wherein the image obtaining unit, the first-information obtaining unit, the second-information obtaining unit, the generating unit, the superposing unit are embodied as a program residing on a non-transitory computer-readable medium.