1. An X-ray CT system, comprising: a couch configured to have a subject mounted thereon 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 from the radiator that have passed through the subject; an image generator configured to generate a first image that represents a first three-dimensional region of the subject and a second image that represents a second three-dimensional region of the subject, based on data detected by the detector; processing circuitry configured to combine, based on coordinates of first pixels in the first image and coordinates of second pixels in the second image, the first pixels with the second pixels on a one-for-one basis within a predetermined range in the rostrocaudal direction in which a translation distance of the couch is not evenly divisible by a pixel pitch in the coordinates of the first and second images; and a synthesizer configured to generate third pixels relative to the combined first and second pixels, and to generate a third image that includes the third pixels.