A first cell having an electrolyte and a pair of electrodes on the surface of the electrolyte with one of the electrodes facing a gas chamber causes, upon receipt of an applied voltage, a current to flow in accordance with the amount of oxygen discharge while discharging oxygen from the chamber. An open-circuit-induced fault is detected in accordance with a current flow change that is caused by the first cell when the voltage applied to the first cell reverts to a reference level after a temporary deviation from the reference level. A second cell generates a signal in accordance with the oxygen concentration in the measurement target gas chamber. An open-circuit-induced fault is detected in accordance with a signal change that is generated from the second cell when the voltage applied to the first cell reverts to a reference level after a temporary deviation from the reference level.