When a group of (pre-processed) projection data is stored into a projection-data storage unit, a Gaussian-based expansion-data creating unit creates a group of Gaussian-based expansion data that is expanded from each of the group of projection data through linear combination based on a plurality of Gaussian functions that is stored by a Gaussian-function storage unit and has different center points. A reconstruction-image creating unit then creates a reconstruction image by using the Gaussian-based expansion-data created by the Gaussian-based expansion-data creating unit, and stores the created reconstruction image into an image storage unit.