The intensity of an ESR signal assigned to oxygen species or lattice defects was increased by photo-irradiation in Nb-doped TiO2. The excess electron was generated in the process of the formation of O- species or lattice defects on the surface layer of the sample. On the other hand, the rise of electric current appeared on the Nb-doped TiO2 by photo-irradiation. This behavior suggests that the generated electron by photo-irradiation is closely correlated to the rise of the current. The niobium-doped TiO2 was found to have the performance of oxygen-sensing. The oxygen-sensing under photo-irradiation was related to the paramagnetic behavior of oxygen introduced. In addition, NO-sensing was also observed for the Nb-doped TiO2. The function for oxygen-sensing was improved by a heat-treatment for the sample of Nb complex-doped TiO2.