PURPOSE: To mount easily a casing in the stage of mounting the casing to the material to be welded in water by guiding the gas overflowing from a gas holding skirt to a discharge path to permit observation from above the water without interference.

CONSTITUTION: A welding torch 1 is attached to a casing 4 and while a shielding gas is ejected from a welding nozzle 3, the casging 4 is successively sunk into water and is mounted to a part 6 to be welded, by which the periphery of the part 6 and an electrode 2 is maintained in a gaseous atmosphere. The shielding gas overflowing from the bottom surface of a gas holding skirt 8 enters the inside of an outside skirt 12 and is guided via a discharge pipe 13 and a discharge hose 15 to the water surface. The sure observation of the relative position of the material 5 to be welded and the casing 4 from above the water surface is thus made possible without rising of foam around the casing 4 and the mounting of the casing 4 is made easy.