A radiotherapy device control apparatus controls a radiotherapy device. The control apparatus includes a reference image creation section, a transmitted image creation section and an affected area position control section. The radiotherapy device includes a therapeutic radiation irradiation device that radiates therapeutic radiation, an imager that generates an image of a subject by using radiation transmitted through the subject, and a drive device that moves a couch, where the subject is arranged, with respect to the therapeutic radiation irradiation device. The affected area position control section judges whether a relative position of the couch with respect to the therapeutic radiation irradiation device is appropriate and changes the relative position of the couch by using the drive device based on a first position and a second position.