A spot welding machine comprising a first electrode (7) and a second electrode (8) opposed to the first electrode, whereby a workpiece, including, e.g., a conductor wire coated with a heat-resistant resin film and a terminal to be welded to the conductor wire, is subjected to electric-resistance welding in a manner such that the workpiece is held between the first and second electrodes. In the spot welding machine according to the present invention, a third electrode (9) is interposed between the first electrode and the workpiece so that one distal-end side face of the third electrode abuts closely against a workpiece engaging end of the first electrode, and that the other distal-­end side face of the third electrode is in contact with the workpiece. The proximal end of the first electrode is connected to a first terminal of AC power source means (20),and the respective proximal ends of the second and third electrodes are connected to a second terminal. The AC power source means supplies a current, used to heat the third electrode and remove the resin coating film by fusion, to a closed circuit having a route essentially passing through the first terminal, first electrode, third electrode, and second terminal, during a predetermined period. After the removal of the coating film, the aC power source means supplies a current, used to weld the workpiece, to a closed circuit having a route essentially passing through the first terminal, first electrode, third electrode or workpiece and second electrode, and second terminal.