PURPOSE: To make the depth of penetration in the central part of plate thickness and at both ends uniform and improve the quality of a melt stuck part by oscillating a welding wire fed into molten slag automatically.

CONSTITUTION: While electricity is conducted to molten slag 11 to heat the same to high temps., a curved welding wire 9 is passed through the inside of a flat tube-like consumable electrode 8 by a wire feeder and is fed into the slag 11. Since the leading end of the wire 9 is melted successively, the weld point of the wire 9 moves to the plate thickness direction of the material 1a, 1b to be welded, and as a whole, it is oscillated at the curving width L of the wire 9. By the oscillation movement of the welding wire 9, the molten pool 12 under the slag bath is agitated, and the weld heat input is made uniform in the thickness direction of the materials 1a, 1b, whereby weld metal 13 is formed at the width of penetration of the depth of penetration roughly equal to the plate ends.