With respect to projection data covering a plurality of sites, in order to create a medical image in which a uniform noise reduction effect is achieved for all sites within the projection data, an arithmetic device (5) determines one or more of proper subsets on the basis of scanning conditions and reconstruction conditions (step 2). Next, the arithmetic device (5) calculates a penalty-term weight for each proper subset on the basis of a detector output weight corresponding to a set element contained in the proper subset (step 3). Then, the arithmetic unit (5) performs an iterative approximation by using the penalty-term weight for each proper subset (step 4).