1. An image inspection apparatus comprising: a first-image obtaining unit configured to obtain a first image generated by scanning a sheet on which a predetermined image has been printed in advance; a scanned-image obtaining unit configured to obtain a scanned image generated by scanning a printed matter produced by printing a second image additionally on the sheet; a second-image obtaining unit configured to obtain the second image; a first registration unit configured to determine a first registration shift amount of the first image based on a difference in pixel values in a first reference area of the first image and the scanned image; a second registration unit configured to determine a second registration shift amount of the second image based on a difference in pixel values of a second reference area of the second image and the scanned image; a combining unit configured to generate a master image by relatively moving the first image and the second image based on the first registration shift amount and the second registration shift amount and combining the relatively moved version of the first image and the second image into the master image; and an inspection unit configured to inspect the printed matter by comparing the scanned image against the master image.