PURPOSE: To prevent deformation of a condenser lens and to obviate increase in beam diameter by forming a nozzle into double construction so that the pressure of an assist gas is not exerted to the condenser lens.

CONSTITUTION: A nozzle 17 made into double-tube construction is provided between a condenser lens 14 and a laser working part. The laser beam 18 condensed by the lens 14 is enclosed by such nozzle 17. An assist gas such as oxygen, nitrogen or argon is supplied into the clearance 20 between the inside and outside walls of the nozzle 17. The assist gas is blown to the laser working part where the beam 18 is irradiated through the annular aperture 21 at the tip of the nozzle 17. The influence of the gaseous pressure on the lens 14 is prevented by such device and therefore the disturbance in the focus of the lens is prevented.