1. A radiotherapy system comprising: a couch for supporting a subject; a rotatable support device disposed around the couch; a treatment radiation irradiating device attached to the rotatable support device; a treatment radiation control device that controls the treatment radiation irradiating device to irradiate the subject when a position of a target in the subject satisfies a treatment radiation irradiation condition of a treatment plan for motion tracking treatment of the target; an X-ray imaging device to perform X-ray imaging which is attached to the rotatable support device and which includes two separate X-ray tubes which apply X-rays to the subject from a plurality of directions while rotating around the subject and two separate X-ray detectors which acquire a plurality of pairs of bidirectional X-ray images corresponding to the applied X-rays; a target recognizing device that recognizes a three-dimensional position of the target in the subject from each of the pairs of bidirectional X-ray images acquired by the X-ray imaging device; and a first CT image generating apparatus configured to: select, out of the bidirectional X-ray images acquired by the X-ray imaging device, ones of the bidirectional X-ray images in which the three-dimensional position of the target recognized by the target recognizing device satisfies the treatment radiation irradiation condition for the motion tracking treatment of the target, and perform image reconstruction on the selected ones of the bidirectional X-ray images to generate a first cone beam CT image.