A transformed image generation unit 32 generates a transformed image by transforming an oblique image. A target region specifying unit 36 specifies a target region indicating the target in the front image based on a feature extracted from a typical portion of a document written on a target in a sample image and a feature extracted from at least a part of the front image. A verification unit 38 determines whether a target shown in the oblique image is the same as a target shown in the front image by verifying the target region in the front image and a region in the transformed image associated with the target region.