An image processor has a gap amount detector which detects an amount of gap between a reference image and a comparative image, a first shading change rate detector which detects a shading change rate on a circumference which is away from a center of the reference image or comparative image by the gap amount, a second shading change rate detector which detects shading change rates within each area increased by the gap amount from the center of the reference image or comparative image until shading change rates within an area up to an edge of the reference image or comparative image are detected, and a shading corrector which generates a pixel value corrected in shading with respect to each pixel in the reference image based on the shading change rates detected by the first shading change rate detector and second shading change rate detector.