A perovskite-type oxide of LaGaO3 doped with Sr and Mg exhibits a high oxide ion conductivity over a wide range of oxygen partial pressures. In this study, the application of LaGaO3-based oxide as the electrolyte of a potentiometric oxygen sensor was investigated. It became clear that silver is the most effective electrode for decreasing the operating temperature. Although a temperature higher than 800 K is required when a conventional Y2O3-stabilized ZrO2 electrolyte is used, an operating temperature as low as 600 K is adequate when La1-xSrxGa0.8Mg0.2O3 at X = 0.1 and 0.2 is used. The open-circuit potential was slightly smaller than that estimated with the Nernst equation. There was agreement with the Nernst equation down to 673 K when Nd-doped LaGaO3-based oxide was used; Moreover, 90% of the response was attained within 15 s at 673 K for the sensor with Nd-doped LaGaO3-based oxide.