1. An X-ray imaging device comprising: an X-ray source that is able to move along a predetermined movement path; a movement unit configured to cause the X-ray source to move along the predetermined movement path; an imaging platform that is disposed to face the X-ray source; a flat-panel X-ray detector that is provided to the imaging platform; a marker that is disposed in the imaging platform; a control unit that causes the X-ray source to move via the movement means and to capture images respectively including the marker from at least two positions; and an image processing unit that calculates a position of an image of the marker in each of the captured images and calculates a slope of a movement axis of the X-ray source with respect to the X-ray detector based on a relative relationship between positions of images of the marker.