In an X-ray CT apparatus, the X-ray tube generates an X-ray beam during a line-orbit scan, and the collimator plate shields a part of the X-ray beam other than the part that contributes to image reconstruction. The part of the X-ray beam which contributes to the image reconstruction is applied to a subject along the body axis of the subject. The two-dimensional detector system detects the X rays in the same conditions during a circular-orbit scan, while the X-ray tube and the subject are rotating relative to each other. The reconstruction device performs back projection, thereby reconstructing an image.