Within an arc extinguishing chamber are disposed a pair of main electrodes so as to cause a spark starting gap to be defined therebetween. A single auxiliary electrode is also disposed so as to cause reignition in those interspaces between the auxiliary electrode and the respective main electrodes at positions distant from said spark starting gap. Further, paths for supplying therethrough a heated gas to the reigniting spark gaps when the feet at one side of the arcs between the respective main electrodes and the auxiliary electrode have arrived at the prescribed points of the auxiliary electrode are provided.