An X-ray image generating unit alternately generates X-ray images each obtained by radiating X-rays having a higher level of energy and X-ray images each obtained by radiating X-rays having a lower level of energy. Further, every time an X-ray image has been generated by the X-ray image generating unit, a dual energy subtraction image generating unit generates a dual energy subtraction image by using the X-ray image that has just been generated and the X-ray image generated immediately preceding the X-ray image.
