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; 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 distortion information indicative of distortion of said shooting target image based on said reduced image; 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.