1. An image processing system for processing an input image comprising a pixel signal train into two levels with respect to threshold values which are elements of a dither matrix, said image processing system comprising: means period calculator means for calculating a mean period of the input image from the pixel signal train of the input image; dither matrix supply means for generating a plurality of dither matrixes; and dither matrix selector means for selecting one of said dither matrixes in response to the calculated mean period; the dither matrix selector means comprising decision means for determining whether a condition .vertline.T-M.vertline.&gt;T.sub.0 is satisfied (where T is a mean period of the input image, M a size of any of the dither matrixes, and T.sub.0 a predetermined threshold value), and means for selecting any of the dither matrixes which satisfies said condition.