Abstract: An electrical-discharge wire cutting machine has upper and lower wire guides by which a wire electrode is guided under tension. The perpendicularity of the wire electrode to a reference machining plane is detected by a wire electrode perpendicularity gage which is supported on a workpiece table and which has upper and lower detectors. The upper wire guide, which is movable, is positioned successively in four positions, and the workpiece table is moved each time the upper wire guide is positioned in one of the four positions. Positions in which the detectors contact the wire electrode are determined when the upper wire electrode is positioned in the respective four positions. Wire guide spans, or distances between the wire guides and the nearby detectors, are computed on the basis of the positions in which the upper wire guide is positioned, the positions in which the wire electrode is contacted by the detectors, and the distance between upper and lower ends of the detectors.