A detector and a measuring method for detecting an air-fuel ratio in an internal combustion engine or the like. The detector has a tubular body having a closed end, a solid electrolyte permeable to oxygen ions which divides the interior of the tubular body into a closed-end section and an open-end section, and electrodes mounted on opposite surfaces of the solid electrolyte. One pair of the electrodes on the opposite surfaces of the solid electrolyte is connected to a circuit for detecting an electromotive force, thus constituting an oxygen sensor. The other electrode pair is connected to a DC power supply, thus constituting an oxygen pump. The oxygen sensor detects the quantity of oxygen in a gas supplied through a gas dispersion member. The direction of flow and magnitude of and electric current in the oxygen pump are controlled on the basis of an output from the oxygen sensor for enabling the gas to have a stoichiometric air-fuel ratio.