PURPOSE: To enable cutting of a metallic material in a thickness direction with excellent dimensional accuracy of cutting in the case of cutting the thick metallic plate by a laser beam to a shape having a sharp angle by cooling the metallic plate from the rear side in the stage of cutting the sharp angled part.

CONSTITUTION: A thick metallic plate 31 to be worked is connected on rollers 33 to an X-Y table 30 moved in X- and Y directions by the command from a pattern control part 35. The laser light 22 from an oscillator 21 is converged by a condenser lens 24 and is irradiated together with auxiliary cutting gas 28 such as O2 or N2 onto the plate 31 then the plate is cut to a target shape when the table 30 moves. If a sharp angled part exists in the shape to be cut in this case, a cooling liquid 38 is ejected in a form of mist from a nozzle 42 by high pressure gas 37 to cool the sharp angled cutting part from the rear in the stage of cutting said sharp angled part, by which the self-combustion effect of the sharp angled part by the laser light is eliminated and the uniformly cut surface is formed in the thickness direction. The cutting with high dimensional accuracy is thus accomplished.