PURPOSE: To perform output control of an excellent responding property in a simple constitution as well as to prevent the laser beam detector from being burned by a method wherein parts of laser beams are uniformly dimmed out using an integrating sphere and the output of the beams dimmed out is detected at high speed by the detector.

CONSTITUTION: Laser beams 14 are outputted from the partial transmitting mirror 13 of a container 7, which has electrodes 9A and 9B and has been filled with medium gas 8, and detecting laser beams 15 are similarly outputted from the partical transmitting mirror 12 of the container 7. The detecting laser beams 15 are inputted from the input port 28 of an integrating sphere 26 and repeat irregular reflections. The laser beams dimmed out fully and uniformly are inputted in a laser beam detector 19 from an output port 29. The output of the detector 19 is compared with an output commanding value 17 and controls the output of a power source 10 through an amplifier 18. According to such a way, as the fully dimmed lase beams are inputted in the detector 19, the detector 19 is prevented from being burned and it becomes possible for the detector 19 to detect the output of the fully dimmed laser beams at high speed, thereby enabling to upgrade the responding property of the output control.