Oxide ion conductivity in La2GeO5-based oxide was investigated and it was found that La-deficient La2GeO5 exhibits oxide ion conductivity over a wide range of oxygen partial pressure. The crystal structure of La2GeO5 was estimated to be monoclinic with P2(1)/c space group. Conductivity increased with increasing the amount of La deficiency and the maximum value was attained at x = 0.39 in La2-xGeO5-delta. The oxide ion transport number in La2GeO5-based oxide was estimated to be unity by the electromotive force measurement in H-2-O-2 and N-2-O-2 gas concentration cells. At a temperature higher than 1000 K, the oxide ion conductivity of La1.61GeO5-delta was almost the same as that of La0.9Sr0.1Ga0.8Mg0.2O3-delta or Ce0.85Gd0.15O2-delta, which are well-known fast oxide ion conductors. On the other hand, a change in the activation energy for oxide ion conductivity was observed at 973 K, and at intermediate temperature, the oxide ion conductivity of La1.61GeO5-delta became much smaller than that of these well-known fast oxide ion conductors. The change in the activation energy of the oxide ion conductivity seems to be caused by a change in the local oxygen vacancy structure. However, doping a small amount of Sr for La in La2GeO5 was effective to stabilize the high-temperature crystal structure to low temperature. Consequently, doping a small amount of Sr increases the oxide ion conductivity of La2GeO5-based oxide at low temperature.