An air/fuel ratio sensor has an oxygen sensor cell including a primary measuring element in direct contact with a gas such as exhaust gases of an automotive engine, and an oxygen pump cell formed by inner and outer electrodes and a cover of an oxygen ion conductive solid electrolyte which confines the primary measuring element within a gas diffusion control space, and which has a small gas diffusion hole. The primary measuring element is formed with a central opening whose center lies right below the gas diffusion hole, in order to reduce a burden imposed on the oxygen pump cell and prevent overshoot in a sensor response characteristic. The oxygen sensor cell may be composed of the primary measuring element of a transition metal oxide connected with a pair of electrodes for measurement of an electrical resistance of the primary element, or may be composed of an oxygen ion conductive solid electrolyte substrate sandwiched between a reference electrode and the primary measuring element which serves as a measuring electrode.