An air-fuel detection system comprising a solid electrolyte, first and second electrodes formed on the surface of the solid electrolyte, a diffusion resistor formed on the first electrode for controlling the gas diffusion into the first electrode, and a detector circuit for detecting conditions of the engine exhaust gas to detect the air-fuel ratio thereof, wherein the detector circuit measures the electromotive force generated between the first and second electrodes during a first period, and controls a selected one of the voltage and current applied between the first and second electrodes to keep the electromotive force constant during a second period, the selected one of the voltage and current being produced as a signal representing the air-fuel ratio.