As regards a first cell, which comprises a solid electrolyte and a pair of electrodes on the surface of the solid electrolyte with one of the electrodes facing a measurement target gas chamber and causes, upon receipt of an applied voltage, a current to flow in accordance with the amount of oxygen discharge while discharging oxygen from the measurement target gas chamber, an open-circuit-induced fault is detected in accordance with a change in a current flow 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. As regards a second cell, which 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 change in a signal 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.