The slice thickness of tomograms of an X-ray CT apparatus is to be kept as constant as practicable within an xy plane. In projected data before three-dimensional back-projection, after convolving row-directional filter of an X-ray detector whose filter coefficient is adjusted channel by channel, three-dimensional back-projection is performed to achieve image reconstruction thereby to control the slice thickness of tomograms according to the distance from the center of the xy plane. The slice thickness is controlled to keep it as constant as practicable independent of the distance from that center to regulate the picture quality of tomograms. Further, the relative density of distances on the reconstruction plane of X-ray detector data or projection data projected on the reconstruction plane is controlled by creating virtual projection data thereby to improve the picture quality of tomograms. |