A radiotherapy apparatus controller includes: a movement collection section; a sensor control section configured to change a first time interval in which a second sensor measures a position of an irradiation area in the subject, based on the movement information; and an irradiation control section. The movement collection section collects movement information indicating a movement of a subject from a first sensor. The sensor control section changes a first time interval in which a second sensor measures a position of an irradiation area in the subject, based on the movement information. The irradiation control section controls a radiotherapy apparatus such that therapeutic radiation irradiated to the irradiation area is changed based on the position.