PURPOSE: To easily execute an optional sealing by leading in gas from plural nozzles, respectively, mixing it by a mixing chamber, and thereafter, feeding it through a feed nozzle to a working part of an object to be worked, together with a laser beam which has been condensed by a condensing lens.

CONSTITUTION: Gas is led into a mixing chamber and mixed through a leading-in hole 19, and annular bodies 20, 21 from leading-in nozzles 11a, 11b which have been separated and installed to a cylindrical body 15. Subsequently, the gas which has been mixed by the mixing chamber 20 passes through an open hole of an end plate 25 and fed to a working part of an object to be worked W from a feeding nozzle 13. From bombs 27a, 27b which have been packed with each component gas, each gas is fed by controlling a pressure and a flow rate by a pressure gauge 28 and a flow meter 29 which have been provided in a feeding passage, and gas of a prescribed mixing ratio is obtained. In this way, optional shield gas can be formed.