An image obtainment unit obtains plural radiographic-images generated by arranging plural storable-phosphor-sheets in such a manner that at least portions of the sheets are overlapped with each other in an irradiation direction of radiation, by arranging a marker at a position to be detected by the plural sheets, and by detecting the radiation that has passed through a subject and the marker by each of the plural sheets. A first-information obtainment unit obtains first-information representing a distance between detection surfaces of the plural sheets. A second-information obtainment unit obtains second-information representing a magnification ratio of a second marker image of the marker included in a radiographic- image obtained by a second sheet with respect to a marker image of the marker included in a radiographic-image obtained by a first sheet of the plural sheets. A distance calculation unit calculates, based on the first and the second information, a radiation-source-to-image-surface distance.
