A radiotherapy apparatus controlling method according to the present invention includes controlling an imager driving unit to move an imaging unit to a position, and to image a transmission image by the imaging unit based on a radiation emitted from a first irradiating unit and controlling the imager driving unit to move the imaging unit to a position, and to image a transmission image based on a radiation emitted from a second irradiating unit. According to such a radiotherapy apparatus controlling method, it is not necessary to separately provide an imaging unit which images the transmission image using a first radiation emitted from the first irradiating unit and an imaging unit which images a transmission image from the radiation emitted from the second irradiating unit, and a radiotherapy apparatus can be manufactured to be cheaper.
