PURPOSE: To detect the theoretical air-fuel ratio stoichiometrically and at the same time to detect the air-fuel ratio in a wide range except the theoretical air-fuel ratio with a simple element structure.

CONSTITUTION: Electrodes 3, 4 are formed on both sides of zirconia 2 and the electrodes 3, 4 are covered with gas diffusion layers 5, 6 with different porosity each other. It is made possible to select either a circuit to apply rated voltage of a rated voltage power source 10 or a circuit to apply rated current of a rated current power source 8 with a switch 7. In the case of detection of the theoretical air-fuel ratio, rated current is applied between the electrodes 3, 4 and the voltage change is detected by a voltage detector 9. On the other hand, in the case of detection of the air-fuel ratio except the theoretical air-fuel ratio, rated voltage is applied to the electrodes 3, 4 and the current value Ip is detected by a current detector 11 and based on the detected current value level, rich or lean air-fuel ratio is detected.