1. An image distortion correction apparatus comprising: a circumscribed rectangle extraction unit which extracts a circumscribed rectangle for each character in a distorted image scanned by an image reading unit to read an original placed on a reference plane; a character string extraction unit which extracts character strings using said circumscribed rectangles extracted by said circumscribed rectangle extraction unit; a distance estimation unit which estimates a distance between said reference plane and said original using said character strings; and an image distortion correction unit which corrects said distorted image based on said distance between said reference plane and said original estimated by said image distortion correction unit.