PURPOSE: To assure stable weldability and welding workability and to obtain weld zones of good quality by oscillating the tip of an electrode in parallel with a wall thickness or groove width in correspondence to the sectional shape of deposited metal.

CONSTITUTION: If the relation between the groove width (w) of respective layers and the wall thickness (h) of deposited metal is w<h in the stage of vertical multi-layer welding of a butt joint having a groove angle by an electrogas welding method, the tip of a welding electrode is oscillated in the direction parallel with the wall thickness (h) of deposited metal. In the case of w>h, the tip of the welding electrode is oscillated in the direction parallel with the width (w). Thus the good weld zones having no weld defects are obtained at specified quantity of weld heat input even if the relation between the width (w) of beads and the wall thickness (h) of deposited metal is not in a one-to-one relation.