1. A method of correcting image distortion, comprising the steps of: obtaining a scanned image having distortion caused by scanning; estimating an average character size of characters in the scanned image; forming one or more circumscribed rectangles for the characters, each of the circumscribed rectangles having a length in a main scanning direction determined by the average character size; extracting one or more character lines based on the formed circumscribed rectangles; and correcting the distortion by using the extracted character lines.