PURPOSE: To prevent projections from adhering to the reverse side of a workpiece, by jetting gas from the processing advance part of the reverse side of the workpiece to the opposite side of processed part and supplying gas from the processing advance part of the obverse side of the workpiece and crossing the direction of the gas supply with that of the gas jet.

CONSTITUTION: Gas is supplied from an inlet port 7 at the laser processing advance part 10a of the obverse side 10 of the workpiece to an exhaust part 34 behind the processed part 4 through the opening 9 of a passage member 6 along the processed part 4. Gas is jetted from the processing advance part 10d of the reverse side 10c of the workpiece to the reverse side 11 of the processed part through a nozzle 24 along an optical axis 5. A portion of the gas supplied flows along the processed part 4 to the rear 10b thereof, and another portion of the gas flows from the opening 9 to the processed part 4 to scatter a molten matter behind the processed part 10b. Another molten matter having flown down is blown up, heated and evaporated by the gas ejected from the nozzle 24 and then scattered together with the exhaust gas