A NOx-decomposing electrode is provided having a certain or a high NOx-decomposing/reducing ability, which is formed on a third solid electrolyte layer for decomposing a NOx to produce oxygen. The detecting electrode contains a noble metal Pt, a material ZrO2 of the third solid electrolyte layer, and a mixture containing silica (SiO2) and alumina (Al2O3). Specifically, the detecting electrode contains 80% to 90% by weight of the Pt, 9.5% to 19.8% by weight of the ZrO2, and 0.2% to 0.5% by weight of the mixture containing silica and alumina.