1. An image restoration method configured to restore an original image from an observed image obtained through a photoelectric conversion of an optical image formed by partially coherent imaging, the image restoration method comprising the steps of: performing an approximate partially coherent imaging operation for elements of a first basis generated from a model image that is assumed to have no noise and no blur, and generating based upon the first basis a second basis that is blurred, the approximate partially coherent imaging operation being expressed by a convolution integral on an eigenfunction corresponding to a maximum eigenvalue of a Kernel matrix and each element of the first basis; generating an intermediate image in which each pixel value of the observed image that has been denoised is replaced with its square root; and obtaining a restored image by approximating each of a plurality of patches that are set to entirely cover the intermediate image, using a linear combination of elements of the first basis and linear combination coefficients obtained when each patch is approximated by a linear combination of the second basis.