PURPOSE: To form a satisfactory penetration bead at the time of one-side welding of materials with high thermal conductivity and different mass by giving the specific relation between the root face and a root gap of a groove and besides, providing a prescribed overhang part.

CONSTITUTION: At the time of the one-side welding of a plug of the material with high thermal conductivity, for instance, nickel and aluminum bronze material, the root face RF2 at the plug 2 side at the smaller mass side is made to 1mm of the same dimensions as those of the root gap RG and besides, the RF1 of a base metal part 1 at the larger mass side is made to 2mm of twice the size of the RG and the overhang part 5 of 2mm of twice the size of the RG is formed and welded. As this result, since the heat is equally inputted to the base metal part 1 side and the plug 2 side, the satisfactory penetration bead 3 can be surely formed even at the time of one-side welding members with high thermal conductivity and besides, difference thermal capacity to each other.