In image data obtained, the radius (50) and the ulna (52), and interosseous soft tissue (78) between the two bones are identified. A midpoint (Ygk) of the length of the interosseous soft tissue (78) in a Y axis direction in (Xk) coordinates is determined. This midpoint is determined in multiple coordinates, and an approximate straight line of these midpoints is determined and set as a reference line (80). The foot of the perpendicular from the ulna styloid process (54) to the reference line (80) is set as a reference position (82). A region of interest is set in a position at a predetermined distance from the reference position (82) along the reference line (80).
