1. A radiotherapy system comprising: a couch for supporting a subject; a rotatable support device disposed to surround the couch; a treatment radiation irradiating device attached to the rotatable support device; and a treatment radiation control device that controls the treatment radiation irradiating device so as to irradiate the subject when the position of a target in the subject satisfies a treatment radiation irradiation condition for motion tracking treatment, wherein the radiotherapy system further comprises: an X-ray imaging device attached to the rotatable support device and configured to apply X-rays to the subject from a plurality of directions while rotating around the subject to perform X-ray imaging; a target recognizing device that recognizes a three-dimensional position of the target in the subject from X-ray images acquired by the X-ray imaging device; and a first CT image generating apparatus configured to select, out of the X-ray images acquired by the X-ray imaging device, the images in which the position of the target recognized by the target recognizing device satisfies the treatment radiation irradiation condition for the motion tracking treatment, perform image reconstruction, and generate a first cone beam CT image.