A radiotherapy apparatus includes an receiver receiving a first projection image of a calibration object under X-ray imaging； a storing portion storing an ideal projection image of the calibration object, the ideal projection image generated based on design information of an X-ray imaging structure, positional information of the calibration object, and volume data of the calibration object； a calculator calculating a transformation parameter for transforming the first projection image into an ideal projection image； a transformed image generator generating a transformed projection image of the patient by transforming a second projection image of a patient obtained under X-ray imaging with the transformation parameter； a reconstructed image generator generating a reconstructed projection image based on volume data of the patient, positional information of the patient, and the design information； and a matching image generator generating a matching reference image used for matching between the transformed projection image and the reconstructed projection image.
