This invention specifies the presence/absence of alteration and an alteration position. A block division unit divides image data into image data of a first unit including a predetermined number of pixels. A block calculation unit calculates an image data coefficient of the first unit. A cell division unit divides one image data of the first unit into image data of a second unit. A cell calculation unit calculates an image data coefficient of the second unit. First verification data is generated by selecting two image data coefficients of the first unit and generating, for one pair, data representing the magnitude relation of the image data coefficients of the first unit. Second verification data is generated by selecting two image data coefficients of the second unit and generating, for one pair, data representing the magnitude relation of the image data coefficients of the second unit.