In this invention, a main image (91) and a sub-image (92) are printed on the surface of a tablet by the ink-jet method. The main image (91) is an image that is visible under ordinary environmental lighting. The sub-image (92) is an image that has a lower visibility under ordinary environmental lighting than the main image (91). Information to be displayed to a consumer is included in the main image (91), while information to be checked by a specific party is included in the sub-image (92). Since it is unlikely that the consumer will be aware of the information included in the sub-image (92), it is possible to avoid confusing the consumer with a large amount of information.
