PURPOSE: To weld securely the superposed parts of a thin copper alloy sheet and an auxiliary sheet of an alloy having the electric conductivity lower than that of said sheet by superposing the latter sheet on the surface of the former sheet, and conducting electricity thereto under pressure while the rear of said thin sheet is held superposed on a thick iron alloy plate.

CONSTITUTION: An auxiliary sheet 4 of an alloy having the electric conductivity smaller than that of a thin material 3 to be welded consisting of either one of phosphor bronze and beryllium copper and having a suitable thickness is superposed on the surface of said thin sheet. On the other hand, the rear of the sheet 3 is superposed on the surface to be welded of a thick iron alloy plate 2 and these sheets and plates are disposed in a superposed state between electrodes 1 and 1' for spot welding. An input is made into a welding machine to conduct electricity to the electrodes 1, 1' under the pressure exerted thereon so that three sheets of the alloy plates including the above-described plate 2, the sheet 3 and the sheet 4 are welded to one another at the contact surfaces thereof. The secure spot welding of the thin copper alloy sheet having large air permeability to the thick iron alloy plate with high quality is thus made possible.