1. A radiation tomographic imaging method comprising: utilizing a processor to execute: a controlling step of controlling a data collection subsystem comprising a multi-slice detector to collect data of a plurality of views 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 of each view 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; wherein the first reconstructing step applies the weighting using a weight function that has a relatively small weight for a region with a relatively large cone angle of a radiation path and a relatively large weight for a region with a relatively small cone angle of the radiation path, and in which a middle of the weight function is coincident with a time of collection of data corresponding to the slice position for which image reconstruction is applied.