PURPOSE: To provide the spot welding machine for welding the nut where the nut is fitted into a through-hole provided on a steel sheet and the nut is welded to the steel sheet by electric spot welding by easily setting a sensor, etc., which can discriminate accurately whether or not the nut is normally welded on an existing spot welding machine.

CONSTITUTION: In the spot welding machine for welding the nut to press-contact and electrify the nut 12 on the steel sheet 10 by energizing an electrically nonconductive guide pin 2 upward to project it from the center of a lower electrode 1, passing the guide pin 2 through the through-hole 11. of the steel sheet 1.0 placed on the lower electrode 1 and putting the nut 12 on the tip of the guide pin 2 to lower an upper electrode 8, a vertical hole 9 where a projecting part of the guide pin 2 is fitted idly is provided on the upper electrode 8, a sensor to detect a lowering state of the guide pin 2 is provided inside the lower electrode 1 and the presence or absence of the nut 12 can be discriminated by an electric signal of the sensor at the time of electrifying.