For the suppression of hole conduction in the high oxygen partial pressure range, effects of the addition of small amount of rare earth cations were investigated in this study with La0.8Sr0.2Ga0.8Mg0.2O3-delta. It was found that the addition of small amount of rare earth cations for La site in LaGaO3 was effective for the suppression of hole conduction under high oxygen partial pressure, albeit the electrical conductivity decreased slightly. In particular, perovskite oxide of (La0.9Nd0.1)(0.8)Sr0.2Ga0.8Mg0.2O3-delta exhibits almost pure oxide ionic conduction over the oxygen partial pressure from 1 to 10(-21) atm, and the electrical conductivity attains the value of log (sigma/S cm(-1)) = -0.5 at 1223 K.