The electrical conductivity of Ce0.992Nb0.008O2−δ was measured as a function of oxygen partial pressure between 10−28 and 1 bar in the temperature range of 873–1173 K. The activation energy for the conductivity at an oxygen partial pressure of 10−4 bar was 0.29 eV. This value was too small to be assigned for oxide ion conduction, indicating that, in oxidizing atmospheres, electronic conduction is dominant due to Nb doping. On the other hand, the oxygen partial pressure dependence of conductivity was complicated in reducing atmospheres. Defect chemical analysis was made to interpret the conductivity variation with oxygen partial pressure. The defect association model was introduced, which assumes that oxygen vacancies are easily formed around niobium ions and that one of the released electrons is trapped on Nb5+. The experimental data was found in accordance with that calculated by the proposed defect association model.