PURPOSE: To prevent the occurrence of a defective product by cutting off automatically the power supply of a welding machine when a material to be welded is not supplied in a multiple electrode spot welding machine.

CONSTITUTION: Movable electrodes 20 exist in the upside of fixed electrodes 15. First sensors 23 exist, which are formed in cylindrical shapes with plastic on the outer peripheries of the lower parts of the movable electrodes 20, and they are slid in the vertical direction. By pushing out the first sensors 23 downward by coil springs 25, the bottom end 23c of a detecting body 23b is projected downward by a prescribed quantity from the movable electrode 20. Movable contact pieces 26 and fixed contact pieces 27 form the contact points of the breaking circuit of a spot welding machine. The guide pin 17-2 of the right part is projected upward by a spring 16 and it stays as it is when a nut 36 is not supplied. By a movement that the guide pin 17-2 collides with a second sensor 30, a first sensor 23-2 of the right part is relatively moved upward to a movable electrode 20-2. It follows that the movable contact piece 26 is detached from the fixed contact piece 27, the power supply of the spot welding machine is cut off, and a nut 36 is not welded on a metallic plate 35.