1. An X-ray CT system comprising: a couch configured to be mounted with a subject and to be translated in the rostrocaudal direction of the subject; a radiator configured to irradiate the subject mounted on the couch with X-rays; a detector configured to detect X-rays that have passed through the subject, of the X-rays radiated by the radiator; an image generator configured to generate an image that represents a three-dimensional region of the subject, based on data detected by the detector; a processor configured to combine, based on the coordinates of a first pixel in a first image that the image generator has generated about a first three-dimensional region of the subject and the coordinates of a second pixel in a second image generated about a second three-dimensional region of the subject, the first pixel with the second pixel on a one-for-one basis within a predetermined range in the rostrocaudal direction; and a synthesizer configured to generate a third pixel relative to the first pixel and the second pixel constituting the combination by the processor and to generate a third image that includes the third pixel.