An information-detecting apparatus (1) including a receiving unit (3) operable to receive digital data (2) that includes several pieces of element data, in which a change amount is imparted to a value of each of first and second element data among the several pieces of element data, a first selecting unit (4) operable to select, as target data (6), a data set that includes the first and second element data, a second selecting unit (5) operable to select, as neighboring data (7), element data in proximity to each of the first and second element data, a calculating unit (8) operable to calculate, based on both of the target data (6) and the neighboring data (7), the change amount imparted to each of the first and second element data, and a detecting unit (10) operable to detect the additional information in accordance with the change amount.