A method of calculating an exposure a patient receives during radiography includes retrieving a value of a dose rate, which is associated with a value of the tube voltage of an X-ray tube, from a first lookup table that is created in advance, retrieving a value of an area of an X-ray field, which is associated with a value indicating the positions of blades included in a collimator that limits a range of X-ray radiation, from a second lookup table that is created in advance, and calculating the product of the dose rate, the area of the X-ray field, the tube current of said X-ray tube, and an X-irradiation time.