1. A radiotherapy apparatus comprising: a first receiver configured to receive a first projection image of a calibration object loaded on a bed which has been moved to an irradiation region of a therapeutic beam of treating a patient, the first projection image being obtained under X-ray imaging; a storing portion configured to store an ideal projection image of the calibration object, the ideal projection image generated based on design information of an X-ray imaging structure which performs the X-ray imaging, positional information of the calibration object in the irradiation region, and volume data of the calibration object; a calculator configured to calculate at least one transformation parameter by which the first projection image is transformed into the ideal projection image; a second receiver configured to receive a second projection image obtained by performing the X-ray imaging on the patient lying on the bed which has been moved to the irradiation region; a transformed image generator configured to generate at least one transformed projection image of the patient which is a resultant image of transforming the second projection image based on the at least one transformation parameter; a reconstructed image generator configured to generate a reconstructed projection image based on volume data obtained by three-dimensionally imaging inside of a body of the patient, positional information of the patient in the irradiation region, and the design information; and a matching image generator configured to generate a matching reference image by which matching processing between the transformed projection image and the reconstructed projection image is performed.