A simple type of zone-plate interferometer has been developed to measure precisely the positioning error of a cutting tool. Interference fringes obtained by the interferometer are little affected by air turbulence in the optical paths and by machine vibrations. The shape of the mirrors surface being tested is spherical and is manufactured with an ultraprecision lathe. A zone plate is set at the midposition between the vertex of the spherical mirror being tested and the center of curvature of the mirror. The error in the shape of the mirror and the positioning error of the tool can be determined by analyzing the interference fringes. Two spherical concave mirrors were measured. One mirror was manufactured with a tool that had some positioning error. The positioning error was observed as distortion of the interference fringes. The images obtained by the zone-plate interferometer agree well with the images obtained by a Fizeau interferometer and a computer simulation based on the experimental results. The other mirror was manufactured with the tool after the positioning error had been eliminated, based upon the results of the above experiments. The interference fringes of the mirror show no distortion, and the error in the shape of the mirror is small.