In order to provide an X-ray CT apparatus and the like that can specify an optimal cardiac phase in a wide variety of cases, an X-ray CT apparatus collects X-ray information and electrocardiographic waveform data 5 by performing cardiac scanning using an scanning unit 1 (step S11). Then, reconstructed images of a plurality of cardiac phases are created (step S12), and a region-of-interest image is generated by extracting a region of interest for each of the reconstructed images of the plurality of cardiac phases (step S13). Then, a variation distribution image is generated by calculating a variation for each region-of-interest image (step S14). Then, the degree of harmony of each cardiac phase is calculated using the variation distribution image (step S15). Then, an optimal cardiac phase is determined on the basis of at least the degree of harmony (step S16).
