1. An image processing apparatus which corrects distortion of an image, and comprises: a shot-image acquisition section which acquires a shot image of a subject to be shot, including a shooting target image which is an image portion corresponding to said subject to be shot, wherein a real shape of said subject to be shot is a rectangle; a reduced-image acquisition section which acquires a reduced image that is said shot image with a reduced resolution; a distortion information acquisition section which acquires a corrected reduced image by performing distortion correction on said reduced image according to a characteristic of a lens used in shooting, acquires a contour of said shooting target image based on said corrected reduced image, specifies four line portions of said contour, and acquires a shape of a trapezoid formed by said four line portions as distortion information; a projection parameter acquisition section which acquires a projection parameter for converting said shooting target image to an ideal distortion-free image based on said distortion information; and a projection transformation section which produces a projection-transformed image by performing projection transformation on said shot image using said projection parameter.