To improve the welding quality by supporting a copper strap to be pressed in the direction to approach a steel plate in a prescribed position to a groove to unify the heat input of each part of the groove section.

A truck 100 is mounted on a rail member and allowed to self- travel in the Z direction. A vertical position welding torch T is supported on the truck 100 by torch supporting mechanisms A, C, and supported in a prescribed position to a groove. A copper strop P is supported in the prescribed position by a strap supporting mechanism G to the groove, and the strap supporting mechanism G is supported on the truck 100. The copper strap P is supported in an advancing/retracting manner in the axial direction parallel to the extension direction Z of the groove, and in the direction orthogonal to the direction Z and while supported turnably around the axis g12a parallel to the plane of a steel plate WL, the strap is further supported advanceably/ retractably in a direction orthogonal to above two axes. The copper strap P is pressed in the direction approaching the steel plate WL, and supported at the prescribed position to the groove. An amt. of cladding by welding is increased, and the durability of the whole joint part can be improved.