PURPOSE: To reduce cutting cost and to improve working efficiency by using high-pressure gas produced by adding H to gas other than O as assisting gas and setting the focusing position on the lower part of the work surface.

CONSTITUTION: A gas bomb 9 wherein H2 is stored, a bomb 10 for Ar or N2, a bomb 11 for O2 and a compressor 12 are arranged to the inside of an assisting gas feeder 8. When cutting is carried out by using stainless steel as a work 7, H2 gas of the bomb 9 is mixed with Ar or N2 gas of the bomb 10 at a prescribed mixing ratio under the high pressure and then, supplied to a nozzle 6 and laser beam cutting is carried out. Further, the focusing position of a laser beam 2 is set on the lower part of the work 7 surface. Since oxidation of the cut surface is prevented by interposition of the H2 gas as a reducer, the single small-sized nozzle meets the requirement. In addition, since the high- pressure gas is used, sticking of dross is reduced and cost is reduced and working efficiency is improved.