An apparatus for superposing a digital watermark for superposing digital watermark information on an information data signal bearing original information, superposes digital watermark information on an information data signal bearing original information, for each unit block consisting of a group of pieces of information data. This apparatus is provided with an auxiliary embedding circuit 10 for embedding second digital watermark information in the information data; a blocking circuit 12 for dividing into the unit blocks the changed information data signal obtained by the auxiliary embedding circuit; and a main embedding circuit 20 for embedding first digital watermark information in the changed information data signal for each of the divided blocks. The second digital watermark information bears information for identifying the position of the divided unit block in a predetermined section of the information data signal. On the basis of the second watermark information, a detecting apparatus effects proper blocking with respect to the information data signal with the watermark embedded therein, so as to detect the first watermark information.