A radiotherapy system acquires an image which is necessary for positioning of a patient for radiation treatment and enables grasping of a positional relationship of a target in a treatment radiation irradiated state, a radiation passing area and a critical organ. An X-ray imaging device is attached to the rotatable support device and configured to apply X-rays to the subject from plural directions while rotating around the subject to perform X-ray imaging. A target recognizing device recognizes a three-dimensional position of the target in the subject from X-ray images acquired by the X-ray imaging device; and CT image generating devices are configured to select, from the X-ray images acquired by the X-ray imaging device, the images in which the position of the target recognized by the recognizing device satisfies the treatment radiation irradiation condition for the motion tracking treatment to perform image reconstruction and generate a cone beam CT image.