An image data forming unit forms image data including an image of an ulna and a radius and an image data identification unit (16) identifies an image portion corresponding to a bone and an image portion corresponding to a soft tissue in the image data. A separation line setting unit (20) sets a separation line that passes through an image portion corresponding to an interosseous soft tissue between the ulna and the radius based on the identified image data, and a bone image separation unit (22) extracts an image portion including the ulna by separating the image portion corresponding to a bone into two parts with the separation line. A styloid process specifying unit (24) specifies an ulna styloid process based on the shape of the image portion including the ulna.