PURPOSE: To obtain an extremely good penetration bead with a method for performing gas shielded arc welding while weaving from the lower side of a groove disposed with a backing strip on the upper side by providing a recess to the backing strip and performing specifically the weaving.

CONSTITUTION: A backing strip 4 is disposed on the upper side of a groove 3 formed between the ends of base metals 1 and 2. A pair of spacers 7, 8 positioned on both sides in the longitudinal direction of a backing strip 4 are adhered to the bottom surface of said backing strip and a recess 9 is consequently formed in the position facing the groove 3 of the strip 4. An overhead welding operation is thereupon performed by shielded gas arc welding. An arc is oscillated, i.e., woven within the horizontal plane parallel with the direction where the welding advances. The arc is so oscillated as to be positioned forward and backward in the advancing direction of the welding respectively near both ends of a bead 10 and near the central part thereof. The formation of the molten metal is consequently accomplished mainly in the central part of the bead and the maintenance of the arc mainly at both ends of the bead, by which the satisfactory overhead penetration welding is accomplished.