Partial electronic and hole conductivity in Co doped LaGaO₃ based perovskite oxide was investigated with the ion-blocking method. Typical S-shaped polarization curves were observed on La_0.8Sr_0.2Ga_0.8Mg_0.2−XCo_XO₃ (0 < X < 0.1). The oxygen partial pressure (P_O2) dependence of the electronic and hole conductivity is estimated to be P_O2^−1/4 and P_O2^1/4, respectively, at temperature higher than 1173 K. However, these decreased to P_O2^−0.12 and P_O2^0.06 respectively at 873 K. It is considered that the electronic and hole conductivities, that are intrinsic to LSGM are dominant at high temperature, however, the extrinsic electronic and hole conductivity caused by doped Co becomes dominant with decreasing temperature. The estimated transport number of the Co doped sample was higher than 0.95 over the P_O2 range from 1 to 10⁻³⁰ atm, which is slightly higher than that estimated by the H₂–O₂ cell. The partial electronic and hole conductivities in Co doped LaGaO₃ based oxide increased with increasing the amount of Co, in particular, increase in the electronic conductivity is significant at Co content higher than 8.5 mol% to Ga site. P_O2 dependence for electronic and hole conductivity is much smaller than that of P_O2^−1/4 and P_O2^1/4, respectively, suggesting that the electronic and hole conductivity which is extrinsic to LSGM is dominant with increasing Co amount and the specimens behaves like an intrinsic semiconductor. The estimated theoretical efficiency of the electrolyte reaches a maximum value of ca. 0.90 around a thickness of 100 μm in 5 mol% Co doped sample at 0.8 A cm⁻² and 1073 K.