PURPOSE: To eliminate the nugget phenomenon of a backup surface so as to provide a substantial electrical conduction and cooling effect with the wide surface area and to reduce the cost of operation by using iron or steel with a surface hardened layer having specific hardness or above for a backup material for spot welding.

CONSTITUTION: The iron or steel with the surface hardened layer having ≥300 Vickers hardness is used for the backup material in spot welding. The nugget phenomenon of the contact surface by conduction of electricity for welding is prevented in such a case. The substantial electrical conduction is assured and the cooling effect is obtd. by the wide contact area thereof. The need for using the conventional copper material is thus eliminated and the consumption of the backup material is decreased, by which the cost of the operation is reduced. The effect of reducing the cost is made much higher if the backup material is cooled with water.