An object is to provide a radiation therapy system which can easily and accurately set an X-ray irradiation range. A movable X-ray diaphragm unit 6 has, in its configuration, an X-ray shielding unit configured with a plurality of members, a driver for the X-ray shielding unit, and a position detector for acquiring a position of the X-ray shielding unit. It has displays 8 and 9 which display a transmission image of a subject based on an output of an X-ray detection unit and a simulation image when the X-ray shielding unit is projected on the X-ray detection unit 2 based on an output of the position detector.
