An object is to more accurately compare diagnosis indexes with each other, which are calculated from data obtained in different environments with a cardiac-function diagnostic medicine. A conversion function calculation unit acquires a first phantom Heart/Mediastinum ratio (H/M ratio) and a second phantom H/M ratio based on a phantom, the first phantom H/M ratio that is an H/M ratio of the phantom in the first imaging environment being acquired by performing, based on phantom data that is data of a first phantom image obtained by imaging the phantom in the first imaging environment and digital phantom data that is data of a digital phantom including a cardiac ROI and a mediastinum ROI, positioning of the digital phantom on the first phantom image, and by calculating based on the phantom data of the first phantom image to which the cardiac ROI and the mediastinum ROI are set； and obtains a conversion function based on the first phantom H/M ratio and the second phantom H/M ratio.
