PURPOSE: To inspect printed matter with high accuracy by certainly extracting a rule becoming a standard judging whether a character is printed within a specific region.

CONSTITUTION: An image to be processed is inputted 1-1 by a CCD to be subjected to density projection 1-2. When the secondary differentiated values of the respective coordinates of a density projection chart are subtracted from the brightness values of said coordinates, the part of the shoulder of an edge is emphasized 1-3. Next, contraction processing 1-4 is applied to a calculated image. In storing processing, for example, the brightness value of a pixel having the min. brightness value within a 3×3 convolution is set to the value of a central pixel. By this method, a rule part can be certainly extracted 1-5 and printed matter can be inspected with high accuracy.