Effects of Ga addition to NdAlO3 on the oxygen ion conductivity were studied. Substitution of Ga to Al sites is effective for enhancing the oxygen ion conductivity of Nd0.9Ca0.1AlO3 and the highest oxygen ion conduction was obtained at x = 0.5 in Nd0.9Ca0.1Al1-xGaxO3. Since the increased oxygen ion conduction by doping Ga suppressed the p-type semiconduction at high oxygen partial pressure, electrical conductivity was almost independent of the oxygen partial pressure in a range from P(O2) = 1 to 10(-20) atm. The oxygen ion conductivity of Nd0.9Ca0.1Al0.5Ga0.5O3 at 1223 K is as high as log (sigma/S cm-1) = -1.4, and furthermore, the oxide ion transference number is estimated to be almost 1.0 over a wide range of oxygen partial pressure and temperature.