An X-ray CT scanner performs CT imaging twice on a single imaging site such that respiratory phases relative to rotation of a measuring unit (X-ray generator and X-ray detector) are inverted with respect to each other. For determining a start time of the second CT imaging, a respiration cycle and the like are calculated according to projection data acquired by the first CT imaging. Further, sinograms 70a and 70b are generated, on each of which sets of projection data acquired from each CT imaging are arranged in the order of rotational angles. Subsequently, a corrected sinogram is generated in which sets of data fluctuated portions b and f attributable to respiration in the first sinogram 70a are replaced with data of the corresponding portions in the second sinogram 70b. Finally, a tomographic image is generated on the basis of the corrected sinogram.