Abstract: A test pattern comprising a half-tone pattern portion and an edge pattern portion printed according to standard printing data is read by a CCD sensor. A pattern discriminator detects the positions of the half-tone pattern portion and the edge pattern portion. In accordance with a prescribed edge detection criteria using the primary differential data .DELTA.D of density determined by primary differential filters, the CPU determines whether the inputted image data correspond to a uniform density region or whether the inputted image data correspond to an edge portion. If the region discrimination result is different from the result of the pattern detection, the edge detection criteria are changed. The image data is then subjected to MTF correction, and conditions for smoothing or edge emphasizing are altered so that the difference between the corrected image data and the standard printing data remains within a allowable range.