The DC current variation of calcia doped zirconia single crystal was measured under various oxygen containing gases at high temperatures. The DC current was influenced by the gas species for oxygen activity establishment. Also, strong non-ohmic characteristics were observed in the $CO/CO_2/N_2$ gas mixtures. Based on the experimental data obtained by introducing the non-buffering gas $N_2$ into the $CO/CO_2$ mixtures, the processes occurring at the gas/solid interface during a defect relaxation process are discussed.