A radiotherapy system is provided that permit acquiring of an image which is necessary for positioning of a patient (1) for radiation treatment and enables grasping of a positional relationship of a target (2) in a treatment radiation irradiated state, a radiation passing area and a critical organ. The radiotherapy system includes: an X-ray imaging device 45 attached to the rotatable support device (7) and configured to apply X-rays to the subject (1) from plural directions while rotating around the subject (1) to perform X-ray imaging； a target recognizing device 9 that recognizes a three-dimensional position of the target 2 in the subject 1 from X-ray images acquired by the X-ray imaging device 45； and CT image generating devices 10, 11, 12 configured to select, from the X-ray images acquired by the X-ray imaging device 45, the images in which the position of the target 2 recognized by the recognizing device 9 satisfies the treatment radiation irradiation condition for the motion tracking treatment to perform image reconstruction and generate a cone beam CT image.
