High precision identification is achieved while reducing the dose by optimizing an identification map automatically in a dual energy X ray CT system. An object composition is identified by applying an X-ray absorption factor acquired in the dual energy X ray CT system to an identification map indicating the relation of the X-ray absorption factor and the composition of an object. An identification map making section calculates the existence probability of each composition based on the photography conditions such as the photographing part of the object, the tube voltage, and the like, for eachcombination of a plurality of kinds of the X-ray absorption factor value and determines a composition of largest existence probability as a composition corresponding to the combination of the X-ray absorption factor value thus making an identification map. Since the identification map can be made for every photography condition, high precision identification can be achieved.