TECHNOLOGY FOCUS - CERAMICS AND GLASS - Preferred Composition: The raw material ceramic powder contains 80-99.99 mass% of electrolyte powder and 0.01-20 mass% of oxide nanoparticles. The electrolyte powder contains perovskite type oxide of stabilized zirconia and/or lanthanum gallate group containing solid solution (3-15 mol%) of oxides chosen from scandium oxide, yttrium oxide, and ytterbium oxide. Preferred Conditions: The oxide particles are milled with a dispersing agent, an electrolyte powder and a solvent are added to the obtained slurry, the mixture is milled, a binder and a plasticizer are added to the obtained slurry, and the mixture is further milled.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Property: The oxide nanoparticles have an electric conductivity of 10-4 S/cm or more at 800 degrees C. Preferred Oxides: The oxide nanoparticles contain zirconia, stabilized zirconia containing solid solution of scandium oxide, yttrium oxide, and/or ytterbium oxide, ceria, doped ceria containing solid solution of gadolinium oxide, samarium oxide, and/or yttrium oxide, bismuth oxide, and/or stabilized bismuth oxide containing solid solution of yttrium oxide, niobium oxide, and/or tungsten oxide.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Agent: The dispersing agent is non-ionic surfactant. The solvent contains 2-4C alkyl alcohol. EXAMPLE - (In mass parts) gadolinia-doped ceria nanoparticles (1) having an average particle diameter of 37 nm and a specific surface area of 42 m2/g was stirred in isopropanol containing sorbitan fatty acid ester (nonionic surfactant). The liquid mixture was milled. The particles in the milled liquid had an average particle diameter of 31 nm. The milled liquid was supplied to a ball mill, and scandia-ceria stabilized zirconia powder (99) having an average particle diameter of 0.5 mu m and a specific surface area of 11 m2/g was added and mixed for 16 hours. A binder (100) consisting of methacrylate copolymer having a mean molecular weight of 80000 and a glass transition temperature of -8 degrees C, was added to the obtained raw material ceramic powder (18). The mixture was mixed with dibutylphthalate and mixed solvent of toluene/isopropanol, to obtain a slurry. The slurry was concentrated. The concentrated slurry had a viscosity of 2.5 Pa.second. The slurry was coated on a polyethylene terephthalate film, and dried to obtain a green sheet. The sheet was baked at 1450 degrees C for 5 hours, to obtain an electrolyte sheet. The sheet had excellent durability, oxygen-ion electroconductivity, high-temperature mechanical strength, and handleability. The 3-point bending strength was 370 MPa and the electroconductivity was 0.11 S/cm.