Image processing apparatus for correcting distortion of image and image shooting apparatus for correcting distortion of shot image

A digital camera (1) acquires the image of an original including an original (2) from a shot image acquired by shooting. The digital camera (1) reduces the image of the original and corrects distortion of the reduced image which is originated from the characteristic of a lens. As the digital camera (1) performs distortion correction on the reduced image, the number of arithmetic operations in distortion correction is reduced, making distortion correction simpler. The digital camera (1) acquires a rectangle defined by the contour of the original (2), extracts the image of the original to acquire an original image before reduction, and acquires an associated projection-transformed image by affine transformation. The digital camera (1) acquires pixel positions of the original image using an affine parameter, and acquires pixel positions of the image before distortion correction using a relational expression of distortion correction performed on the reduced image.