1. A degradation restoration system that generates a restored image based on an optimization function which includes a plurality of regularization terms, that reduce variation in pixel value of the restored image toward different directions assigned to the individual regularization terms, and an error term where the smaller the difference between an input image and an image obtained by virtually degrading the restored image in accordance with a pre-assumed degradation step, the less the value of the error term, wherein the system comprises: a variation computation unit that computes, for each pixel of the input image, a direction toward which variation in pixel value between the relevant pixel and its peripheral pixels is largest and an amount of the variation in pixel value between the relevant pixel and its peripheral pixels, where the variation computation unit computes a direction unit vector that indicates, for each pixel, the direction toward which the variation is largest and a variation vector that represents, for each pixel, the amount of the largest variation between the relevant pixel and its peripheral pixels; a regularization strength computation unit that computes, for each pixel, a regularization strength utilizing the direction unit vector, the variation vector, and a regularization strength computing formula in a manner such that the greater the degree of coincidence between a direction toward which each regularization term reduces the variation in pixel value and a direction of the variation in pixel value, which is indicated by the direction unit vector computed by the variation computation unit, and also the larger the amount of the variation in pixel value, the less the regularization strength; and an image restoration unit that determines the optimization function based on the input image and the regularization strength assigned to each pixel of the input image and assigns a value for each pixel, by which the determined optimization function has a minimum value, to the corresponding pixel of the restored image so as to generate the restored image.