In an image scan reader, a white reference plate is attached to a transparent plate on which an original is to be placed. Different regions are defined on the reference plate and respective optical densities on a plurality of scanning lines (L.sub.Al -L.sub.An, L.sub.Bl -L.sub.Bm) are detected and averaged within each region (R.sub.A, R.sub.B). The maximum averaged densities are selected for each given pixel position to be used for correcting the non-uniformity of CCD cells which are employed in reading the white reference plate and the image of the original.