Main image information representing a portrait face image of a person is prepared.
Predetermined pattern image information, used as sub image information, is also
prepared. Synthetic image information is prepared by synthesizing the main image
information with the pattern image information in such a manner that the pattern
image information is embedded in the main image information in a state that is
not recognizable to the eye of a human being. This synthetic image information
is recorded on an ID card or the like as a visible image.
