A lanthanum gallate-based sintered body comprises a base material formed of lanthanum gallate-based oxide, and grains having a K.sub.2NiF.sub.4 structure and/or grains having a layered perovskite structure are dispersed into the base material. Consequently, the lanthanum gallate-based sintered body has a high mechanical strength and is excellent in thermal resistance. Further, a SOFC having excellent properties can be obtained by using the lanthanum gallate-based sintered body as the solid electrolyte.