An oxygen sensor is disposed behind an exhaust gas purifying catalyst for an internal combustion engine and which suppresses the influence of unburnt hydrocarbon on the output voltage. A sensor element 1 is provided with a detection electrode 4 on the outer face of a zirconia ceramic body 2, a reference electrode 3 on the inner face of the ceramic body 2 and a spinel protective layer 5 on the outer surface of the detection electrode 4. The tip end of the sensor element 1 is covered with a protective cover 10 which is provided with an outer partition wall 11 having through holes 11a and an inner partition wall 12 having through holes 12a. In a case where the pressure difference between the inside and outside of the protective cover 10 is .DELTA.P(atm) when the atmospheric air with a volume flow rate Q(1/min) is passed into the protective cover 10 from the outside, the ratio .DELTA.P/Q2 is 3.2.times.10 -5(atm.cndot.min2.cndot.1 -2) . When the surface temperature of the sensor element 1 is changed in a range of sensor active temperature, the reversing time is substantially constant. Therefore, the catalyst deterioration can be detected by the oxygen sensor with high accuracy.
