An apparatus for processing a two-dimensional image obtained by scanning sensor arrays of image detectors perpendicular thereto, the apparatus including an image memory for storing pixel data output by the image detector as a two-dimensional image, a first read-out unit for reading the stored pixel data about a particular detector in a particular array, a second read-out unit for reading pixel data in the neighborhood of the pixel data read by the first read-out unit, a first arithmetic unit for measuring the magnitude of difference in sensitivity between the detector whose pixel data is read by the first read-out unit and the detector whose pixel data is read by the second read-out unit, and a second arithmetic unit for compensating for the deficiency of sensitivity on the basis of the output of the first arithmetic unit.