An image processing apparatus is provided for processing a plurality of sets of projection data acquired by radiating an X-ray onto an object in a multitude of directions. The apparatus has a correcting unit and a reconstructing unit. The correcting unit corrects the projection data with regard to beam hardening of the projection data. The beam hardening is caused due to a contrast agent injected into the object. For example, the correcting unit includes a correction table defining a correcting value to a change in densities of a region in which the contrast agent is present and corrects the projection data on the basis of the correcting value obtained from the correction table. The reconstructing unit reconstructs the corrected projection data into an image of the object. Thus, artifacts due to beam hardening resultant from use of the contrast agent can be avoided.