1. A method of correcting image distortion, comprising; using a processor to perform the steps of: obtaining a scanned image assumed to have distortion caused by scanning; extracting a pixel run for each character from the scanned image, the pixel run caused to have a selected ranged length in a main scanning direction forming a circumscribed rectangle for each character based on the extracted pixel run having the selected ranged length, the circumscribed rectangle representing a vertical component of the character; extracting a character line based on the circumscribed rectangles, the character line including the circumscribed rectangles for one or more of the characters that are arranged in a same line; and correcting the distortion by using the character line, wherein the forming further includes: adjusting the length of the circumscribed rectangle in the main scanning direction to be within a predetermined value, wherein the character line extracting includes: defining a coordinate of each of the circumscribed rectangles in the character line, with the coordinate being a central point of the circumscribed rectangle in the main scanning direction; and defining coordinates of the character line based on the coordinates of the circumscribed rectangles, wherein when the coordinate of the character line is located relatively closely to a coordinate of the character line that is adjacent with the character line, the length of at least one of the circumscribed rectangles in the character line is reduced without changing the coordinate of the at least one of the circumscribed rectangles.