An acquisition unit acquires image data resulting from reading an image printed sequentially in a first direction. A setting unit sets a division size in the first direction and a second direction of a division area for dividing the image data, the second direction crossing the first direction, as well as a shift amount in the first direction and the second direction of the division area. A processing unit performs a predetermined process including an averaging process on the image data in accordance with the division size and the shift amount. An extraction unit extracts a unique portion from image data on which the predetermined process has been performed. The setting unit sets (i) the division size greater in the second direction than in the first direction, and/or (ii) the shift amount greater in the second direction than in the first direction.