Ceria doped yttria stabilized zirconia (ZrO2-CeO2-Y2O3 system) shows electron-ion mixed conduction at high temperatures under a reduced atmosphere. In this system, the electronic conductivity is caused by electrons, which may diffuse by hopping conduction between Ce4+ and Ce3+. We applied the electronic Raman scattering technique on the system in order to measure the concentration of Ce3+ in samples. For the X = 0.6 and 0.8 samples in [(ZrO2)1-X (CeO2)X]0.9(Y2O3)0.1, the oxygen partial pressure dependence of the electronic Raman intensities agrees well with that of the oxygen non-stoichiometry measured by thermogravimetry. From this result, it is confirmed that all of the introduced electrons at reduced condition are trapped at cerium centers in these materials.