When image reconstruction by retrospective scanning is performed, the cache hit ratio is lowered due to transfer of not-used projection data to a cache memory. Accordingly, a processor generates rearranged projection data in which a first plurality of segments are extracted from original projection data written in a memory for saving original projection data 205, and stores the generated rearranged projection data into a main memory 212. The processor generates second view weights in which first view weights are made to correspond with the rearranged projection data (209). The rearranged projection data stored in the main memory 212 is previously transferred to the cache memory 213. The processor generates first tomographic image data with the rearranged projection data and the second view weights (214).
