PURPOSE: To enlarge an allowable stroke and to execute laser beam machining in a wide range in an optical scanning type laser beam machine.

CONSTITUTION: A moving condensing system 8 is composed of a beam diameter correcting lens 20, reflecting mirror 5 and condenser lens 6 and their positions are controlled by command from a CNC 100 as represented by arrows 9. The beam diameter correcting lens 20 is provided short of the reflecting mirror 5 and its position is controlled by the command from the CNC 100 as represented by the arrow 21. Thus, even when the moving condensing system 8 is moved and the beam diameter D1 on the condensor lens 6 is changed due to its movement, the beam diameter D1 is held at a fixed value by controlling the position of the beam diameter correcting lens 20. That is, even when the stroke of the moving condensing system 8 is enlarged from the nearest point S3 to the farest point S4, laser beam machining in a wide range is executed while the uniform machining quality is maintained.