In image data obtained, the radius and the ulna, and interosseous soft tissue between two bones are identified. A midpoint (Ygk) of the length of the interosseous soft tissue 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. The foot of the perpendicular from the ulna styloid process to the reference line is set as a reference position. A region of interest is set in a position at a predetermined distance from the reference position along the reference line.
