To provide a long life of a spark-discharging gap always keeping a stable discharge start voltage by preventing insulation deterioration between electrodes from vapor and particulate of electrode material generated during electric discharging.

One side of opposing electrode surface 20 is a flat surface. Another electrode surface 20a is a short gap and long gap combination type provided with a two stage structure high at a center and low at circumference. The long gap exists in a range affected by ionization action during electric discharging at the short gap. The opposing electrodes are fixed on a porcelain attachment stand 26. A centre electrode forming the short gap is directly shaved out from a main electrode or combined with an outside electrode with using material of higher heat resistance than the outside electrode material or of stable electric discharging voltage. Especially, the short gap interposes thin plate (for example, mica) 32 of high heat resistance and high insulation and is press contacted to opposing electrode to stick to a pair of opposing electrodes. Plural cut edge parts are added on each opposing surface of the outside spark gap.