To stabilize laser focusing by arranging plurality of bypass sensors, selecting automatically from the signals of these sensors the most suitable sensor for machining when a torch traces a machining route, and performing the copying control of the torch axis (Z axis) based on the signal of the sensor so selected.

The basis of selecting one among plurality of sensors is such that (1) the sensor is closest to or furthest from the work, that (2) the nozzle gap calculated from the signal of sensors is closest to or furthest from a target value, that (3) the gap calculated from the sensor signal is most approximate to or most away from the arithmetically obtained gap converted from the Z axis position and the work setting height, and that (4) the sensor for which the Z axis position inversely operated by the calculated gap from the sensor signal is closest to or furthest from the Z axis position at the time of entering a copying area immediately preceding. As a result, a means is employed such that a closer sensor is selected for machining the upper face of a projected part and that a further sensor is selected for machining the wide area of a recessed part.