A three-dimensional CT image is obtained and output by sequentially storing image data of an image of an object to be processed according to a prescribed direction, and using a multicore processor, performing a back projection/projection process in parallel by a plurality of threads so that each thread sequentially processes in a direction that is orthogonal to the prescribed direction. Accordingly, by using a multicore processor such as a GPU, there is high flexibility and less restrictions and the technical problem of the process speed being limited by the band width is solved. At the same time, when processing the data using a multicore processor in storage and back projection/projection process, by taking into account the rules for multithread memory access of a multicore processor such as a GPU, cache hit ratio is improved, coalesced access is achieved and memory access speed is greatly improved.
