PURPOSE: To prevent the crack of a welding metal by forming a space between the joining faces of a 1st and 2nd metals and performing a welding with acting the tensile force into the weld zone direction from this space on the molten metal.

CONSTITUTION: 1st and 2nd metals 1, 2 are subjected to a butt arrangement by forming a space 7 between joining faces 5, 6 and turned over so that weld zones 3, 4 may become underneath. In this state, the space 7 is weld zones 3, 4 may become underneath. In this state, the space 7 is located to the upper part of the weld zones 3, 4 to perform the welding of weld zones 3, 4. In this case, a gravity G is acted for the welding metal in the weld zones 3, 4 directions from the space 7 side. A molten metal 8 is therefore dropped down and the lower face 11a becomes round with the surface tension. The upper face 11b of the molten metal 8 is recessed as well in a semi-spherical shape by the dead weight, gravity G and surface tension. Consequently a stress concentration part is not formed on the molten metal 8 after the solidification and the crack generation on the welding metal can be prevented.