An apparatus (10) includes a device for acquiring a plurality of images of an identical subject taken from a plurality of viewpoints; a device for selecting a prescribed image as a reference image, selecting an image other than the reference image as a target image from among the images, and detecting feature points from the reference image and corresponding points from the target image to generate pairs of the feature point and corresponding point, wherein feature of the feature point and the corresponding point in the same pair are substantially identical; a device for estimating geometrical transformation parameters for geometrically-transforming the target image such that y-coordinate values of the feature point and the corresponding point included in the same pair are substantially identical, wherein y-direction is orthogonal to a parallax direction of the viewpoints; and a device for geometrically-transforming the target image based on the parameters.