1. A tomographical image reconstruction method for reconstructing an image of a subject from a plurality of sets of digital projection data respectively acquired from the subject at different projection angles in an angle range, said method comprising the steps of: in a processor, forming a first analytical filter kernel using a first analytical function, for a filtered back projection in a spatial frequency domain, that approximates, at least in a frequency range of said spatial frequency domain, a discrete filter kernel iteratively determined for a model; and from said sets of digital projection data, reconstructing an image of the subject by filtered back projection operating on said sets of digital projection data with said first analytical filter kernel and with a second analytical filter kernel calculated from said first analytical filter kernel and formed by a second analytical function, to obtain a set of image data representing the reconstructed image of the subject.