An X-ray CT apparatus is adapted for switchover between a CT mode to obtain a tomographic image of a subject and a transmission mode to obtain a transmission image of the subject, depending upon projection data acquired in an imager system thereof. The apparatus includes an image-data storage device that stores image data of the subject in a three-dimensional region, a three-dimensional image processor that performs a three-dimensional image processing on the image data and generates three-dimensional image data, for display, corresponding to a line-of-sight direction inputted from an input device, and a transmission-image generator that generates the transmission image as to the image-taking direction, from data acquired in the imager system aligned by the CPU.