1. A geometric feature extracting device comprising: first input means for inputting a three-dimensional shape model of a measurement object; generation means for generating two-dimensional parameter planes corresponding to curved surface patches that configure the three-dimensional shape model; first calculation means for calculating normal directions to points on the curved surface patches; holding means for holding the parameter planes and the normal directions in association with each other; second input means for inputting an observation direction used to observe the measurement object from an observation position; selection means for selecting regions in each of which the normal direction and the observation direction satisfy a predetermined angle condition from the parameter planes; and second calculation means for calculating geometric features on the three-dimensional shape model corresponding to the regions selected by said selection means as geometric features that configure geometric feature regions on the three-dimensional shape model, which are observable from the observation position.