A real-time tracking radiotherapy system includes, a therapeutic radiation irradiation device (4), at least two X-ray imaging devices (5A； 5B), a target position recognizing section, a therapeutic radiation irradiation control section, each X-ray imaging device (5A； 5B) including an X-ray, an X-ray detector (9A； 9B), and X-ray variable diaphragms (15A； 15B), a transit region calculation section, a radiography range setting section, and a diaphragm control section. The real-time tracking radiotherapy system is capable of appropriately setting a radiography range using a computed transit region of a target (3) or an surrogate marker and achieving a reduction in the amount of X-ray exposure.
