An offset data calculator has a normal offset data calculation function for calculating offset data based on data obtained from an X-ray flat panel detector while no X-ray is incident in each of the modes, i.e., an entire region fluoroscopy mode, a partial region fluoroscopy mode, imaging mode, etc. and storing the data in an offset data storage device, and additionally has a function for updating, upon calculation of new offset data in any of the modes, another mode offset data stored in the offset data storage device based on the new offset data calculated. Thus, from offset data acquired in any of the plurality of modes, it is possible to obtain the latest offset data in all the modes.