1. Three-dimensional measurement method, comprising the steps of: acquiring, by an arithmetic unit, a stereo image of a work taken by first and second cameras; extracting, by the arithmetic unit, edges from the acquired stereo image; selecting, by the arithmetic unit, edges existing on a same plane of the work from the edges extracted on the stereo image; three-dimensionally restoring, by the arithmetic unit, at least two first edges among the edges existing on the same plane by a stereo method, the first edges residing in a predetermined angle range with reference to a cross angle of 90° made by crossing with an epipolar line; calculating, by the arithmetic unit, a three-dimensional plane including the three-dimensionally restored first edges using the three-dimensionally restored at least two first edges; and three-dimensionally restoring, by the arithmetic unit, a second edge among the edges existing on the same plane as the first edges by projecting the second edge on the three-dimensional plane, the second edge residing beyond a predetermined angle range with reference to a cross angle of 90° made by crossing with an epipolar line.