To express a code, which is used as an electronic watermark in order to specify a wrong entity while conspiring and attacking, with a smaller number of bits.

Plural digital contents acquisition entities arranged into an n×n square are divided into groups in vertical, horizontal, and oblique directions, and codes which can indicate a group to which a wrong entity belongs in the case of alteration of a code padded as a watermark are respectively given to groups Ai, Bi, and Ci to which one entity belongs, and individual codes given to these groups are connected to generate a code peculiar to the entity, thereby expressing the code peculiar to the entity as the combination of codes of groups of which the number of bits is considerably smaller than that required for giving codes to individual entities as conventional. As the result, the code peculiar to the entity is expressed more shortly that conventional.