An apparatus for reducing the influence by scatter rays and improving image quality of a tomographic image. The apparatus includes an X-ray detector and a calculation/control apparatus for generating tomographic image data for a tomographic image of the subject based on the projection data. The X-ray detector includes a plurality of detector channels for detecting the radiation, extending in a two-dimensional manner in two arrangement directions, i.e., in channel and column directions, the channel direction being contained in a plane of rotation of the X-ray source, the column direction being orthogonal to the channel direction and aligned along the axis of rotation; and collimators for confining an angle at which X-rays impinge upon the detector channels, provided at borders between the detector channels adjoining in the column direction.