1. An image processing apparatus, comprising: a processor; an acceptance unit, operable on the processor, that accepts image data and reading data read by an image reading apparatus from a medium on which an image of the image data is formed by an image forming apparatus; an extraction unit, operable on the processor, that extracts rectangular images by segmenting one image of the image data and the reading data to have a size based on a tolerable error which occurs from the image forming apparatus and/or the image reading apparatus, the tolerable error being predetermined in design specification of the image forming apparatus and the image reading apparatus; a first searching unit, operable on the processor, that searches for a matching position of a selected rectangular image in the other image of the image data and the reading data within a search range, the selected rectangular image being selected from at least one of the rectangular images, and the search range being set from the tolerable error; an estimation unit, operable on the processor, that estimates a matching position of a rectangular image in the other image on the basis of the matching position of the selected rectangular image in the other image; a second searching unit, operable on the processor, that searches for a matching position of the other rectangular image in the other image using the matching position estimated by the estimation unit as a reference; and a comparison unit, operable on the processor, that compares the rectangular image extracted by the extraction unit with a corresponding rectangular image in the other image.