A mask-image-taking-time calculating section 36 (i) sets a first imaging-time, in a first mask image, in accordance with X-ray imaging-conditions, and (ii) calculates a second imaging-time, in mask images from the second, for a mask image to be taken next in accordance with the imaging-time and brightness of the mask image taken previously such that average brightness of the mask image taken previously and a mask image to be taken next is target brightness. A live-image-taking-time calculating section 38 calculates an imaging-time for a live image in accordance with an actual imaging-time for the mask image having X-rays applied thereto from an X-ray irradiating device in accordance with the first or second imaging-time. An image processor 6 calculates a subtraction image by difference between a reference mask image obtained through averaging two or more mask images taken in accordance with the first or second imaging-time and the live image.
