An image region mapping device captures a blood vessel through which a contrast medium is passing, serially at first and second photographing angles to acquire plural image sets each including first and second projection images captured at the angles, respectively acquires brightness change information on the medium for a predetermined time period in a first image region and each of a plurality of second image regions after a bifurcation on the first projection image each of a plurality of second image regions after the bifurcation on the second projection image in each image set, the second image regions being candidates corresponding to the first image region, calculates a similarity degree between the information acquired earlier and each information piece acquired later, determines one of the second image regions in accordance with the calculated similarity degrees, and maps the plural vessel image regions.