A V groove Wv is formed at the abutting portion between the aluminum alloy vertical wall panels Wu and Wl supported side by side, and the angle θa formed by the upper groove surface Wa with respect to the horizontal plane is formed by the lower groove surface Wb with respect to the horizontal plane. The first pass when setting the angle θb to be larger than the angle θb and passing the welding torch 7 multiple times while weaving along the V groove Wv to pile up the meat is performed with the welding torch 7 horizontal, and thereafter, the lower side. The path for filling the meat directly on the groove surface Wb is carried out by tilting the welding torch 7 downward by an angle αb with respect to the horizontal plane, and the path for filling the lower bead on the lower groove surface Wb is the welding torch 7. The welding torch 7 is tilted upward by an angle αa with respect to the horizontal plane in the path in which the weld torch 7 is piled up from the intermediate bead piled up on the lower bead to the upper groove surface Wa. Sideways butt welding can be performed on aluminum alloys without causing welding defects.
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