PURPOSE: To greatly improve machining speed by having a substrate surface irradiated with a laser beam through the innermost tube of a double conical tube, making an assist gas flow between the innermost tube and the outer tube and blowing it to the substrate surface.

CONSTITUTION: The wavelength of a beam generated by a carbon dioxide laser device is selected through grating and controlled into a single wavelength. The laser beam with the single wave length is converged with a lens and emitted to the surface of the substrate 18 through the inner tube 17b of the double conical tube. The diameter of the inner tube 17b is 1 to 3.5mm, and a distance between the end of the exit and the surface of the substrate 18 is made 1mm or more. An assist gas with the pressure of 1kg/cm2 or higher is made to flow between the inner tube 17b and the outer tube 17a and blown to the surface of the substrate 18. By separating the passage of the laser beam transmission from that of the assist gas, the heat energy density is held high on the area of the diameter (d) on the surface of the substrate 18 irradiated with the laser beam; and the outer circumference is cooled by the assist gas so as to bring in the steep distribution of heat energy density.