An NOx sensor for an exhaust gas is made from beta -Nb2O5 as a primary component and TiO2 as a subsidiary component. The TiO2 content is in a range of 0.1% by weight </=TiO2 </= 20% by weight. In place of TiO2, Ru may be used in the NOx sensor. The Ru content is in a range of 0.1% by weight </=Ru</= 10% by weight. Thus, the NOx sensor has an excellent NOx adsorbing ability and is higher in sensitivity to NOx in an exhaust gas.
