To perform the welding which is very immune against the undercut and humping by securing the penetration with the arc heat generated by a preceding wire-shaped electrode in the welding direction, forming the initial molten pool, using a succeeding strip-shaped electrode in the welding direction for a long time, and forming the long molten pool in the welding direction.

A wire-shaped electrode 4 and a strip-shaped electrode 5 are arranged so that their tips are submerged in the flux 6. The initial molten pool is formed with the arc heat generated at the wire-shaped electrode 4 moving in a preceding manner in the welding direction. Because an initial molten pool 7 is formed on a bottom part of a groove prior to the melting by the strip-shaped electrode 5, the penetration is secured, and the slag 8 generated in the initial melting is floated on the molten pool 7 to allow the molten metal to be brought into direct contact with a base metal 1. The succeeding strip- shaped electrode 5 is used for a long time in the welding direction, and a long molten pool 9 is formed in the welding direction with the heat of the arc generated at the strip-shaped electrode 5.