Ammonium gas sensor

An ammonium gas sensor comprising:

• a solid electrolyte layer having oxygen ion conductivity;
• a detection electrode formed on one surface of the solid electrolyte layer;
• a reference electrode that is a counter electrode of the detection electrode;
• a selective reaction layer covering the detection electrode;
• and
• a protection layer covering the selective reaction layer and made from a porous material;
• wherein
• the detection electrode includes a noble metal as a main component;
• the selective reaction layer includes an oxide represented by AxMyOz as a main component, where A is one or more kind(s) of metal, M is vanadium, tungsten, or molybdenum and x, y, z are atomic ratios;
• the protection layer includes the oxide of the selective reaction layer in an amount smaller than a content of the oxide included in the selective reaction layer, and contains at least one selected from the group consisting of MgAl2O4, SiO2, SiO2/Al2O3, zeolite and SiC as a main component;
• and
• the detection electrode is exposed to the gas to be measured.