To cut a thick steel plate with laser beams.

In this laser cutting method and its device, laser beams LB oscillated from a laser beam oscillator 1 is introduced into a straight part 25 having a small diameter and multiply reflected to uniformize beam strength of the laser beams, a diameter of the laser beams the beam strength of which is uniformized is slightly enlarged and the surface of a work W is irradiated with the laser beams, an oxidation promoting gas jet the flow rate in the central part of which is kept at a nearly sound velocity is ejected toward the central part of an irradiating surface by the laser beams, the work is molten by heat owing to an oxidation reaction of the work, and the molten part is removed by the gas jet. A laser nozzle is provided with a tapered opening part 23, the straight part 25 having the small diameter through which the laser beams LB are multiply reflected, and an irradiating port side tapered part 29 the nozzle irradiating port side of which becomes large in diameter gradually so that the diameter of a nozzle irradiating port 27 becomes larger than the diameter of the above described straight part 25.