Abstract: An air/fuel ratio detector providing an unambiguous output for both fuel-rich and fuel-lean regions is disclosed. The detector includes a solid-electrolyte oxygen-concentration-difference-actuated electrochemical cell sensor element and a solid-electrolyte oxygen pump element, each element being in the form of an oxygen-ion-conductive solid electrolyte having a porous electrode formed on both sides thereof. The electrochemical cell sensor element is disposed to face the pump element with a small gap therebetween. An air compartment open to the atmosphere is formed on the side of the pump element opposite the small gap.