5 n 7 The film and the PS plate are read in separately by a scanner to recognize the positional shift therebetween. Based on the quantity of the recognized positional shift, the alignment in position is conducted between the pair of image information. For the alignment in position, the address conversion is applied most preferably as the method. Both the images are compared in such a condition as being aligned in position, and the comparisons are made one by one for fixed regions. The whole image is divided into a plurality of images having a fixed area. Next to this, for each divided region, a judgement is made on whether each divided region is formed of halftoned portion or text. As a result, if a divided portion is determined to be a text, a fixed threshold level for binarizing is set up. When a divided portion is judged as a halftoned portion, a threshold level is set up, what is called dynamically, in accordance with variations of the halftone dot densities. Then, based on the set-up threshold levels, each portion is binarized. After these binarizing operations, all regions of the film divided are respectively compared with the corresponding regions of the PS plate to detect any mismatched portions. It is most preferable to compare the numbers of black (or white) dots since this method agrees with the human's sensation. As a result of the disagreement detection, a judgement is made on whether or not each divided portion disagrees with the corresponding portion more largely than a certain level, if it is so, the region in question is determined as a mismatched one and the error information is set up.