1. A radiation tomographic imaging method causing a computer to execute: a controlling step of controlling a data collection subsystem comprising a multi-slice detector to collect data for a subject by performing a helical scan; a first reconstructing step of reconstructing a plurality of tomographic images at an identical slice position and at different times using data obtained by applying weighting to the collected data according to the slice position, the reconstruction being performed for a plurality of slice positions; a fragmenting step of fragmenting each of the plurality of tomographic images at each of the plurality of slice positions into a respective plurality of local-region images; and a difference identifying step of, for each combination of a plurality of local-region images at the same position and at different times, identifying a difference among local-region images in the combination.