It can be enabled an accurate diagnosis of osteoporosis by a simple method utilizing panoramic radiographs without requiring special skill or experience of a technician engaged in diagnosis of osteoporosis. It comprises an object region image acquisition means for enhancing the contrast of a mandible bone on digitized panoramic radiographs and extracting a portion of a mandible including a mental foramen and a mandibular base as an object region image; a discrimination and selection means for discriminating between a background portion and the portion of a mandible from the object region image and acquiring, as a judgment region image, a portion from which the background portion is removed; a defined image acquisition means for defining forms of a cortical bone and a cancellous bone constituting the mandible of the judgment region image; a means for specifying the cortical bone portion on the image in which the form is defined; calculation means for determining the thickness of the cortical bone based on the outline constituting the mental foramen and the cortical bone specified by the respective means; and an output means for outputting the determined thickness of the cortical bone.