An X-ray image acquiring unit, by irradiating a blood vessel through which contrast medium is passing with X-rays of equal intensity at first and second photographing angles, acquires first and second X-ray images. An absorption property acquiring unit acquires, from brightness of the contrast medium, an X-ray absorption property in a first image region corresponding to a portion of the blood vessel in the first X-ray image. An absorption property acquiring unit acquires, from brightness of the contrast medium, an X-ray absorption property of each of plural second image regions corresponding to a portion of the blood vessel in the second X-ray image as candidate corresponding image regions of the first image region. A similarity degree calculator calculates a similarity degree between the acquired absorption properties. A corresponding region determiner determines the second image region having the maximum similarity degree as a corresponding region of the first image region.