Abstract: Spark gap apparatuses are provided at the receiving end or the sending end of an electric power substation, in order to protect electric apparatus in the electric power substation against the surge voltage incoming from the power transmission lines. In the case where the gap of such a spark gas apparatus is placed in electronegative gas, for example SF.sub.6 gas, the spark voltage vs. spark time characteristic varies depending on the polarity of the surge voltage and thus the protecting performance of the spark gap apparatus is poor. The improvement in the protecting performance of the spark gap apparatus is provided by substantially equilizing the maximum electric field developed between the end surfaces of a pair of electrodes which are oppositely disposed to each other to form a spark gap therebetween.