To reduce labor and to improve working efficiency by oscillation- driving a torch with a controller and supplying welding power.

A copying roller y7 facing to the corner, at which two steel plate W1, W2 are orthogonally crossing, is supported with a first arm y1. A welding torch T is supported with an oscillating mechanism 100, the welding torch T supported with the first arm y1 is oscillated in the direction orthogonal to a rail member 51. An electric motor M1 executes oscillation driving. Electric power is supplied from a welding power source PS to the torch T. A welding wire is supplied with a wire supplying device WS. A controller C energizes two electric motors M1, M2 for driving of a truck and oscillating driving of torch T and supplies welding power to the torch through the welding power source PS. By this method, a welded joint having uniform bead shape and high quality is obtained.