Radiotherapy apparatus controller and radiotherapy apparatus control method

A radiotherapy apparatus control method includes: collecting a planning-stage position where a patient is arranged during a planning stage; collecting a position-adjusting-stage position where the patient is arranged during a position-adjusting stage; calculating a treatment-stage position, a gantry-angle offset value and a ring-angle offset value such that an orientation in which an irradiation head is arranged with respect to the treatment-stage position, when a ring is arranged while deviating from a ring angle by the ring-angle offset value and a gantry is arranged while deviating from a gantry angle by the gantry-angle offset value, coincides with an orientation in which the irradiation head is arranged with respect to the planning-stage position, when the ring is arranged at the ring angle and the gantry is arranged at the gantry angle. After that, the patient is arranged at a patient position.