1. A computer-implemented method of correcting image distortion, comprising the steps of: obtaining a scanned image using a scanner, the scanned image having distortion caused by scanning; estimating an average character size of characters in the scanned image; determining a run length based on the average character size of the characters; extracting one or more pixel runs from the scanned image, each of the pixel runs having a run length in a main scanning direction equal to or larger than the determined run length; forming one or more circumscribed rectangles for the characters based on the extracted and pixel runs; extracting one or more character lines based on the formed circumscribed rectangles; and correcting the distortion of the scanned image by using the extracted character lines, wherein the estimating step comprises: specifying a central area of the scanned image, where the central area includes a plurality of sections in the scanned image; forming one or more circumscribed rectangles, each of the circumscribed rectangles representing a character from the central area; calculating an average length, which represents an average of lengths in a main scanning direction of the circumscribed rectangles, where the average length is estimated for each one of the plurality of sections; and selecting one of the average lengths having a smallest non-zero value, and using the value of the selected average length as the average character size.