PURPOSE: To keep the height of torch constant even when skins, rusts, or paints are stuck on the surface of a working member to be machined.

CONSTITUTION: A torch 3 and a laser sensor 5 are fit to a supporting body 6 which is driven in elevating and lowering with a lifting mechanism 4 by keeping a positional relation being preliminarily set. A reflecting plate 9 having a definite reflection factor is arranged oppositely facing to the laser sensor, and the reflecting plate is fixed on a carriage 13 which has a rotary body 12 and energized to the surface side of the working member B to be machined with a spring 14. A laser beam irradiated from the laser sensor is reflected by the reflecting plate, and a signal is generated from the laser sensor having received the reflecting beam. The height of the torch can be kept by operating the lifting mechanism corresponding to this signal. Because the laser beam is reflected with a reflecting plate, the influence due to the surface state of the working member to be machined is not received.