An X-ray imaging device includes an X-ray generator, a filter plate detachably attached to an X-ray outlet of the X-ray generator, and a FPD. The filter plate has a plurality of circular markers of different sizes. The smallest marker is disposed at the center of the filter plate. The other markers are disposed on lines radiating from the smallest marker in increasing order of size and at regular intervals. An X-ray radiation beam passes through the markers and patients body, and is incident upon an imaging surface of the FPD. The FPD produces a preliminary radiographic image from the incident X-ray radiation beam. A deviation vector detector chooses adjoining two marker images of different sizes from the preliminary radiographic image, and identifies to which markers the marker images correspond based on a size ratio. Then, the deviation vector detector determines the center of an X-ray field.
