Abstract: Radiotracer is dosed to an object to detect radiation rays emitted from the radiotracer. The detection is performed with R waves of an electrocardiogram serving as a trigger at the end of diastole of the myocardium and the end of systole of the same during a predetermined number of heart beats. Detected radiation-ray data is used to reconstruct a SPECT short axis image (an ED image) at the end of the diastole and a SPECT short axis image (an ES image) at the end of systole, each of which is formed of several slices. All slices of the SPECT short axis image are developed on a polar coordinate to make functional maps (bull's-eye map). The amount of shift of the pattern of the segment value of the functional map of the ED image and the segment value of the functional map of the ES image is obtained for each slice by calculating the sum of products. The segment value for each slice is rotated on the functional map by the obtained amount so that the influence of the twist is corrected.