PURPOSE: To surely spot weld a metal material having insulating film without removing the insulating film.

CONSTITUTION: Between welding electrodes 1a, 1b arranged in spot welding machine, heating electrodes 4a, 4b are arranged. The heating electrodes 4a, 4b are energized to be heated. Pressurizing by the electrodes 1a, 1b is executed, with sandwiching a metal material 7 between the heating electrodes 4a, 4b, after the insulating film of the metal material 7 having insulating film is melted, welding current is energized between the welding electrodes 1a, 1b.