An X-ray imaging system is for irradiating a subject with X ray from an X-ray source at a plurality of different angles to acquire X-ray images, and reconstructing an X-ray tomographic image from the X-ray images. The system comprising a region setting unit for setting a region of interest on a pre-shot image or a previously acquired X-ray tomographic image, an imaging control unit for continuously varying an aperture formed by collimator blades according to a position of the X-ray source to image the region of interest and acquiring projection data of the X-ray images in accordance with the position of the X-ray source, and an image reconstructing unit for reconstructing the X-ray tomographic image from the projection data of the X-ray images of the region of interest. The aperture formed by the collimator blades are varied so that all the X-ray images formed on the X-ray detector is rectangular regardless of the position of the X-ray source.
