An image processing system in which, in order to easily analyze input images acquired by sensors, output image quality is improved so that is suitable for a user. It includes: a gradient calculation unit that calculates a desired gradient based on input images; an indication function calculation unit that calculates an indication function for the input images, the indication function defining a range that can be taken by an output image and pixel values of a reference image; a pixel value renewal unit that renews pixel values of one of the input images so as to approximate the desired gradient to produce a renewed image; and a pixel value constraint unit that updates pixel values of the renewed image so as to fall within the range that can be taken by the output image and to approximate the pixel values of the reference image, to thereby obtain the output image.