1. An X-ray image processing method, comprising: providing an imaging unit that supports an X-ray generation unit, which emits an X ray to which a subject is exposed, and an X-ray detection unit, which detects the X-ray that has passed through the subject, such that the X-ray generation unit and the X-ray detection unit face each other and such that it is possible to take an image of the subject placed on a top panel of a bed; performing control such that a step movement process of at least one of the imaging unit and the top panel is carried out and images of the subject are taken at a plurality of stages; setting a plurality of regions of interest for a plurality of locations in both legs of the subject when an image is taken after a contrast medium is injected into a lower limb of the subject; measuring a change of an image level in the regions of interest to detect a flow of the contrast medium and making at least one of the imaging unit and the top panel move to the next imaging stage on the basis of a change in the image level of a leg in which the contrast medium flows more slowly; and processing image data taken at the plurality of stages.