1. An image data correction method for correcting image data picked up from an original image photographed in an exposure frame on a photographic film through a taking lens while holding the exposure frame curved along a direction of the exposure frame, so as to eliminate image distortion in the original image, said image data correction method comprising the steps of: setting up an orthogonal coordinate system having an origin at an intersection between an optical axis of the taking lens and the exposure frame, the X-axis of said coordinate system extending in the curved direction of the exposure frame, and the Y-axis of said coordinate system extending perpendicularly to the optical axis of the taking lens; approximating distortion in an image obtained by photographing straight-linear lines extending in the X-axis and Y-axis directions through the taking lens to circular arcs, wherein distortion in a first straight-linear line extending in the Y-axis direction is approximated to a circular arc which have a center of curvature on the X-axis and passing across an intersection of said first straight-linear line with the X-axis, and distortion in a second straight-linear line extending in the X-axis direction is approximated to a circular arc which have a center of curvature on the Y-axis and passing across an intersection of said second straight-linear line with the Y-axis, said circular arcs having different radii depending upon distances of said respective straight-linear lines from the origin; and rearranging the image data of respective pixels of the original image by transforming coordinates (x, y) of each pixel to coordinates (a, b), wherein “a” represents an x-distance of an intersection between the X-axis and one of said approximating circular arcs that has its center on the X-axis and passes across said coordinates (x, y), and “b” represents a y-distance of an intersection between the Y-axis and one of said approximating circular arcs that has its center on the Y-axis and passes across said coordinates (x, y).