1. A system for measuring space width of joint, comprising: a joint image input unit which inputs an image of a joint; a contour detection unit which detects contours of a pair of bones constituting the joint; a measurement range setting unit which sets a measurement range of a space of the joint based on the contours detected by the contour detection unit; a space width calculation unit which calculates a space width which is indicated by a distance value between the pair of bones in the measurement range set by the measurement range setting unit as a distance value between two points at which a predetermined straight line intersects with the contours of the pair of bones in the measurement range; and a display unit which displays the space width calculated by the space width calculation unit.