Abstract: Upon measuring the image-quality of an original document, the original document is photographed, and the resulting image is displayed on cells within a window so that image-quality data, such as the degree of edge-irregularity a, is obtained. A regression line is found based upon the correlation between the cell size and the image-quality data. By setting the image-quality data resulting from a specific sell size as a reference value, respective pieces of image-quality data are converted and corrected based upon the regression line using the reference value as a target. Thus, it becomes possible to compare pieces of image quality data in an equivalent manner even if they were obtained from measurements using different magnifications.