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 plurality of space widths each of 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 plurality of predetermined straight lines arranged in a predetermined rule each intersects with the contours of the pair of bones in the measurement range as a space width at each position; and a display unit which displays at least one of an average value, a median value, a minimum value, and a graph display of space widths based on the plurality of space widths calculated by the space width calculation unit.