the surface of the gas detecting portion 3 as illustrated in fig1 determination of the characteristics of the nitrogen oxide sensor manufactured as above is carried out as follows a predetermined potential is applied to the heating plate 2 to heat the gas detecting portion 3 to 250 u00b0 to 450 u00b0 c and a predetermined current is applied to the current applying electrodes 4 under these conditions the sensor is exposed to gas containing a predetermined concentration of nitrogen oxide in the air and a potential is obtained from the potential detecting electrodes 5 and an electric resistance developed in the gas detecting portion is obtained according to experiments conducted in the above described manner the sensor having the above construction achieved the afore described distinguished performance tendency in the sensitivity selectivity reversible sensitivity and durability a control sensor having up to 20 of ca substituted for by y in the above described formula achieved substantially the same performance as well as a result there has been obtained a practical and useful nitrogen oxide sensor which is satisfactory in all of the sensor requirements of sensitivity selectivity reversible sensitivity and durability a in the foregoing embodiment the gas detecting portion is provided in the form of thin film instead this portion may be provided in the form of sintered body though may be inferior in the sensitivity performance in comparison with the film type detecting portion the sintered body type gas detecting portion too can achieve substantially same durability performance substantially free from deterioration.