1. An image processing system comprising: converting means for converting a multi-tone-level image into an M-tone-level image using M-1 threshold values, where M-1.gtoreq.2, said M-tone-level image including white pixels, black pixels and middle-tone-level gray pixels; image producing means for treating said middle-tone-level gray pixels in said M-tone-level image as unknown pixels, and giving tone levels other than middle tone levels to said unknown pixels based on states of pixels in a neighborhood of said unknown pixels so as to produce an N-tone-level image, where M&gt;N.gtoreq.2; and wherein all pixels other than pixels of an image to be detected are given tone levels of white pixels, and continuity of known pixels in the image to be detected is determined from pixels in the neighborhood of the known pixel.