When a single X-ray image is formed by compositing a plurality of partial X-ray images, level differences among partial images are corrected. The level difference values at the boundary portions among partial images are statistically calculated using information of an image captured before the partial images to be corrected, and calculating correction values. An image is corrected using the calculated correction values. A system for calculating the correction values, and a system for correcting the partial images are built and driven independently.