A gas sensor includes a second oxygen detection electrode (5) sensitive only to oxygen on a solid electrolyte substrate (3a) and ready for direct contact with a gaseous sample. The electromotive force (signal output 2) appearing between a reference electrode (7) and the oxygen detection electrode (5) is detected as the oxygen concentration of exhaust gas, and the output is compared with a signal output (1) corresponding to the electromotive force between a first oxygen detection electrode (4) and the reference electrode (7) or with the signal output of oxygen pump current, thereby checking for trouble with the sensor.
