A radiotherapy apparatus includes an irradiation head section, an X-ray source section and a sensor array section. The irradiation head section irradiates therapeutic radiation to a therapeutic field of a target substance. The X-ray source section irradiates diagnostic X-rays to the therapeutic field of the target subject. The sensor array section detects the diagnostic X-rays which have transmitted the target subject, and outputs diagnostic X-ray image data based on the detected diagnostic X-rays. The sensor array section moves in conjunction with movement of the irradiation head section.