A radiotherapy device controller includes: a visual line calculator configured to calculate a visual line based on a position on a transmissive image imaged using a radiation penetrating a subject, a specific part of the subject being shown at the position a position calculator configured to calculate a position of the specific part based on a position of a trajectory of the specific part and a position of the visual line and an emitter configured to control a radiation emitter that emits a therapeutic radiation, so that the therapeutic radiation penetrates at the position of the specific part.
