There is provided an air-fuel ratio sensor with which an improvement in the accuracy in detecting the air-fuel ratio of detection target gas and an improvement in the response characteristics can both be achieved. The sensor includes a sensor element that outputs an output signal indicative of the air-fuel ratio of a detection target gas, a pair of electrodes including a detection target gas side electrode to which the detection target gas is introduced and an atmosphere side electrode exposed to the atmosphere, which are arranged in such a way as to sandwich the sensor element, a diffusion-controlling layer that is disposed on the sensor element in such a way as to cover the detection target gas side electrode and introduces the detection target gas from an entrance portion through which the detection target gas flows in to the detection target gas side electrode, and a catalyst layer provided on a part of the entrance portion.