1. An image region mapping device configured to map a plurality of image regions of a blood vessel, the device comprising: an X-ray image acquiring unit configured to, by irradiating the blood vessel through which a contrast medium is passing with X-rays at first and second photographing angles different from each other, acquire an X-ray image set including a first X-ray image at the first photographing angle and a second X-ray image at the second photographing angle; a first X-ray absorption property acquiring unit configured to acquire, as an absorption property, an absorbed X-ray amount in a first image region corresponding to a portion of the blood vessel in the first X-ray image, from brightness of the contrast medium; a second X-ray absorption property acquiring unit configured to acquire, as an absorption property, an absorbed X-ray amount in each of a plurality of 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, from brightness of the contrast medium; a similarity degree calculator configured to calculate a similarity degree between the absorbed X-ray amount acquired by the first X-ray absorption property acquiring unit and each of the plurality of absorbed X-ray amounts acquired by the second X-ray absorption property acquiring unit; and a corresponding region determiner configured to determine one of the second image regions corresponding to the first image region in accordance with the similarity degrees calculated by the similarity degree calculator.