A method of calculating an exposure, which a patient receives during radiography, without detection of X-rays. A radiography system includes an imaging unit that images a patient by utilizing X-rays and a detection unit that detects an exposure the patient receives during radiography. The detection unit comprises: a first lookup table in which the values of a dose rate are recorded in association with the values of a tube voltage of an X-ray tube a second lookup table in which the values of the area of an X-ray field are recorded in association with the values each indicating the positions of blades (222, 242) included in a collimator (20) and a calculator that retrieves the value of the dose rate, which is associated with the current value of the tube voltage, from the first lookup table, retrieves the value of the area of the X-ray field, which is associated with the current value indicating the positions of the blades (222, 242), from the second lookup table, and calculates the product of the dose rate, the area of the X-ray field, the tube current of the X-ray tube, and an X-irradiation time.
