Through the present invention, three-dimensional coordinates of a tracking object moving in an irradiation object can be calculated from fluoroscopic X-ray images captured and acquired from various angles in a single fluoroscopic X-ray device mounted to a radiation therapy apparatus. The three-dimensional coordinates of the tracking object are calculated on a straight tracking object presence line connecting an X-ray generating device for fluoroscopy and the position in an X-ray plane detector of the tracking object on the fluoroscopic X-ray image acquired by the X-ray generating device for fluoroscopy and the X-ray plane detector, and using line segment information included in a movement region of the tracking object set in advance (S104).
