(Ba1-xLax)(2)In2O5+x, whose end member is Ba2In2O5, is an oxygen-deficient perovskite oxide showing high oxide-ion conductivity. In order to clarify the reason why the high oxide ion conductivity appeared in this system, the electrical conductivity was measured as a function of temperature and La content. With an increasing La content, the discontinuous jump of ion conductivity in the Arrhenius plot, which is related to the disordering of the oxygen vacancies, disappeared for the sample with x=0.2. Above x = 0.12, the ion conductivity linearly increased with La content, while the activation energy remained constant with respect to the La content. Moreover, the conductivity for x = 0.6 was 0.042 (S/cm) at 1073 K, which exceeded that of 8 mol% yttria-stabilized zirconia. The higher oxide-ion conductivity of this system could be dominated by the amount of mobile oxygen ions.