Abstract: A gas detecting means of this invention is provided with a discharge section which radiates light by electric discharge, a detecting section which collects an ion current of a sample gas ionized by the light, and an optical path which connects the discharge and the detecting sections. The discharge is formed by a pointed cathode and an anode having a penetration hole. The tip of the pointed cathode is directed to the optical path. The diameter of the penetration hole of the anode is smaller than that of the optical path. The anode is disposed in such a manner that any peripheral part of an inlet of the penetration hole is at an equal distance from the tip of the cathode. Both the discharge section and the detecting section are shielded electrically by an outer shell of metal.