1. An X-ray imaging device comprising: an X-ray generator for applying an X-ray radiation beam to an object to be viewed; a position detection pattern disposed between the X-ray generator and the object to be viewed, for partly passing the X-ray radiation beam therethrough; a flat panel detector disposed behind the object to be viewed, for producing a preliminary radiographic image from the X-ray radiation beam having passed through the position detection pattern and the object to be viewed and being incident upon an imaging surface of the flat panel detector; and a deviation vector detector for determining a center of an X-ray field of the X-ray radiation beam from the preliminary radiographic image, and for calculating a deviation vector extending from the center of the X-ray field to a center of the imaging surface of the flat panel detector.