An La.sub.2O.sub.3 powder and an SiO.sub.2 powder are mixed with each other, and then heated. By heating, a porous material of La.sub.XSi.sub.6O.sub.1.5X+12 (8.ltoreq.X.ltoreq.10) as a composite oxide is produced. Subsequently, the porous material is pulverized to obtain a powder, and the powder is added to a solvent to prepare a slurry. The slurry is solidified in a magnetic field to prepare a compact. After that, the compact is sintered, and an oxide ion conductor is obtained thereby.