NOx SENSOR

A NOx sensor comprising:

• one or a plurality of solid electrolyte bodies which have oxygen ion conductivity;
• a pump electrode which is disposed on a surface of the solid electrolyte body which is exposed to measurement gas containing oxygen, the pump electrode being used to regulate an oxygen concentration in said measurement gas;
• and
• a sensor electrode which is disposed on the surface of the solid electrolyte body which is exposed to the measurement gas and is used to measure a concentration of NOx in the measurement gas whose oxygen concentration has been regulated,
• wherein a metallic component of the sensor electrode is a Pt—Rh alloy,
• wherein a mass ratio of Pt to Rh in a whole of the sensor electrode is Pt:Rh=70:30 to 35:65, and
• wherein a percentage of Rh in the Pt—Rh alloy in a surface layer ranging from an outer surface of the sensor electrode to a depth of 350 nm is higher than a percentage of Rh in the Pt—Rh alloy in a whole of the sensor electrode by an atomic composition percentage of 4 to 10 atom %.