JPH033553B

PURPOSE:To protect treatment efficiency from degradation due to the fluctuation of concentration of by-product by detecting the concentration of the by-product generated from a work during laser beam machining and controlling the volume of the air in a pumping means according to a detected output. CONSTITUTION:The change of concentration of hydrogen sulfide, by-product developed in a circulating chamber 7 due to the fluctuation of the output is detected successively by a densitometer 13 of hydrogen sulfide and inputted into a control device 15 through a signal line 14. An exhauster 12 is controlled by comparing a detected output with a set value. As a result, the exhauster 12 controls variably a displacement so that the concentration of hydrogen sulfide in the circulating chamber 7 is kept within a certain range. Accordingly, the absorption of laser beam by the atmosphere is kept from changing and the machining accuracy is maintained.