To secure good and stable quality of a welded portion capable of perfectly replacing circumferential air of the welded portion with gas while fully controlling a supply amount of gas forming the welding atmosphere, and also controlling invasion of external air without producing any flow of gas during welding operation.

When welding a butt portion 6 of cylindrical members 1 and 2 as an object to be welded by a laser spraying unit 22, the inside of a shielded case 7 is replaced with argon gas so that argon gas 23 having heavier specific gravity than that of air is supplied into the shielded case 7 surrounding the above-described butt portion 6 through a lower part of the shielded case. Then, upon switching supply of argon gas 23 to supply of argon gas 25 by means of a gas delivering feeding portion 16, argon gas 25 is horizontally injected in the upper portion of the shielded case 7. Argon gas 23 having heavier specific gravity than that of air is caused to stay inside the shielded case 7, and invasion of air into the shielded case 7 is eventually controlled.