PURPOSE: To output a signal corresponding to the change of a detection signal between the tip of a machining head and a work and to prevent defective machining and damage of the machining head by sampling the detection signal between the tip of the machining head and the work and comparing it with each set value.

CONSTITUTION: When the electrostatic capacity (detection signal) between a nozzle 4 and a head 9 changes suddenly, a sampling circuit 10 judges whether or not the rate of change thereof exceeds a first specified value 10a and when it exceeds the specified value, the circuit 10 outputs signals to a laser beam oscillator 1 and a servo-amplifier 6 so as to stop the laser beam output and driving of the machining head 3. When the rate of change is less than the first specified value 10a, it judges whether or not the rate of change exceeds a second specified value 10b and when the rate of change per unit time of the detection signal 5a exceeds the second specified value 10b, the sampling circuit 10 holds a signal before sampling of the sampled detection signal 5 and outputs it to a control circuit 5c via an inversion amplifier 5b. The control circuit 5c outputs a control signal to the servo-amplifier 6 so that a compared result attains zero.