1. An image distortion correcting apparatus comprising: an image input section to input an image of a flat rectangular paper surface imaged by an imaging section, as an input image; an imaging position estimating section to estimate a relative imaging position of the imaging section with respect to the paper surface from four vertexes of the rectangular paper surface within the input image; a rectangular paper surface estimating section to estimate four vertexes of the rectangular paper surface within a three-dimensional space based on the imaging position; and an image correcting section to correct a perspective transformation distortion in the paper surface within the input image based on the imaging position and the four vertexes within the three-dimensional space, so as to output an output image.