Abstract: An exhaust gas side electrode is provided on one surface of a solid electrolytic substrate. A reference gas side electrode is provided on an opposite surface of the solid electrolytic substrate so as to be exposed to a reference gas stored in a reference gas chamber. Each lead of the electrode is connected to a signal output terminal. This sensor satisfies a relationship B/A<0.5, wherein ‘A’ represents an overall resistance value of an electric path including the solid electrolytic substrate, the electrodes, and their leads in a sensor activated condition, while ‘B’ represents a resistance value of the leads at a room temperature. Some embodiments may be arranged such that at least one of the leads has a low resistance portion located in the vicinity of the electrodes and a high resistance portion located in the vicinity of signal output terminals.