A gas detector for detecting the decomposed SF.sub.6 gas produced by discharge in gas-insulated equipment. The gas detector operates as a cell generating voltage in proportion to the amount of the decomposed SF.sub.6 gas wherein the voltage is generated between the detection electrode including Ag, reacting upon contact by the decomposed gas and the opposing electrode including Ag also, both electrodes sandwiching the ionic conductive solid electrolyte layer including Ag ion therebetween.