PURPOSE: To detect the theoretical air/fuel ratio of the oxidative and deductive species especially in the exhaust gas from an automobile using advantageous parameters different from conventional ones, e.g. variation in the work function of a metal and resultant variation in a thermal emission ions.

CONSTITUTION: The air/fuel ratio sensor brings an emission electrode 11 of a metal, e.g. platinum, doped with an alkaline metal into thermal contact with a ceraric heater 15 while insulating electrically from a collecting electrode 12. The electrodes 11, 12 are disposed on the opposite sides of a peripheral gas phase retention space with the electrode 11 being sustained at a positive potential with respect to the electrode 12. Tons 16 emitting thermal ions are collected at the collecting electrode 12 and the magnitude of thermal ion current I is measured by an ammeter 14. Interaction between the emission surface of the electrode 11 and the gas phase causes the vary the work function of the emission surface reversibly as the quantitative relationship between the oxidative and deductive species in the gas phase varies through the theoretical air/fuel ratio and the theoretical air/fuel ratio in the gas phase is detected based on the variation of the thermal ion current I.