A control device sets first points on characters, generates a first frame composed of a first point and first circles, attaches first marks to points at which the first circles intersect characters, detects a range having a largest central angle and no first marks, sets second points on the first circles in the detected region, generates a second frame composed of a second point and second circles, attaches second marks to points at which the second circles intersect characters, sets a direction passing through the center portions of ranges having no second marks and the second point, sets second points arranged in the direction as the same class, calculates an approximate line connecting second points for each class, obtains straight lines indicating a row direction of characters immediately above and below an approximate line, and determines an inclination angle of an image from inclinations of the straight lines.