A radiotherapy system includes X-ray imaging apparatuses that obtain an X-ray image of the patient on a reference plane, and a position determination apparatus. The position determination apparatus calculates parameters of a region estimation model, using, as input data, a reference fluoroscopic image obtained before radiotherapy, and also using, as teacher data, a reference ROI image obtained with respect to the reference fluoroscopic image before radiotherapy. During radiotherapy, the position determination apparatus estimates a region of interest with respect to the X-ray image and a DRR image, based on the parameters and the X-ray image, determines a degree of matching between the X-ray image and the DRR image for the region of interest while virtually changing a relative position/orientation relationship between a CT image and the reference plane, and determines an amount of deviation in position/orientation between the patient and the CT image.