After synthesizing particles by liquid phase synthesis, the solution is substituted without drying these particles, and here, a solution comprising a grain boundary phase composition consisting of at least one or more types selected from a group consisting of Al2O3, yttrium oxide, silicon oxide, yttrium-silicon complex oxide, aluminum-silicon complex oxide, and a compound having a garnet structure with a lower melting point than the aforementioned particles, or a solution comprising a precipitate is introduced. Microparticles are adjusted by allowing adhesion and growth of the solution comprising a composition of grain boundary phase or the solution comprising a precipitate on the surface of the particles; these microparticles are allowed to align in 3-dimensions in solution and are formed into a molded body, and this molded body is sintered.