PURPOSE: To detect the air to fuel ratio in excellent manner by detecting the voltage at the junction between air ro fuel ratio sensor and the dividing resistor at specified position of either rich or lean and changing the electric resistance of the dividing resistor depending on this value.

CONSTITUTION: The air to fuel ratio sensor 20 is provided with the exhaust manihold 14 of the engine 10 of spark iginition type, it is connected to the control unit 30 and the air to fuel detector is constituted by giving electric signal to the mixed gas feeder 12. When the air versus fuel ratio A/F is at lean side greater than the theoretical air to fuel ratio ST, the lean resistance is L, and if rich position smaller, the rich resistance R is obtained. This electric resistance characteristic varies as the curves X and Y with the temperature is use or time aging. The electric resistance of the air to fuel ratio sensor 20 is changed with the contents of exhaust gas discharged from the engine 10, especially with the concentration of oxygen, the electric resistance value Re is changed with the air to fuel ratio A/F. The air to fuel ratio detection can be made with good accuracy even if the electric resistance is changed due to the temperature un use and time aging of the sensor.