PURPOSE: To optimize an energy level and to improve processing quality by controlling a laser beam device in accordance with the energy value of the laser light extracted by using a partially reflecting mirror, etc. just before a condenser lens.

CONSTITUTION: A wire chopper 8 swivels a wire 8c at a high speed. The laser light 2a sampling a part of the laser light 2 reflected to the position A of the wire 8c face is irradiated to a sensor 8 which detects the energy level of the laser light 2 in the position A and displays the energy level on a power meter 10. The cause for the decrease in each section from a laser oscillator 1 to a reflecting mirror 5 is traced when the value of the energy level displayed on the power meter 10 decreases beyond the prescribed permissible value. The cause is eliminated and the processing by the laser light in the normal state is executed. The good processing quality is thus obtd.