1. A degraded-image restoration method comprising: inputting a degraded image degraded by a predetermined degradation process; reading from a storage unit a filter coefficient in a filtering process of restoring the degraded image, in which an iteration formula for minimizing a square error between the degraded image and an image obtained by degrading a restored image by the degradation process is expanded corresponding to a predetermined number of iterations; and performing the filtering process using the filter coefficient with respect to the degraded image to obtain a restored image of the degraded image.