To eliminate causes which are responsible for impairing a presentable appearance and deteriorating the quality and the physical properties due to cracking or a drop in the strength of a finished product by filtering slurry of raw material powder to remove a foreign matter regardless of its kind.

Slurry of raw material powder to be used in a cast molding process is prepared by adding a liquid dispersion medium such as an organic solvent, e.g. water or ethanol, an additive and the like to the raw material powder and mixing these materials using a ball mill. The slurry filtering process is such as normal temperature filtering, suction filtering, pressurized filtering or vibration filtering. A filter medium to be used is metal netting, netting of an organic material such as nylon, ceramic filter and the like. The mesh apertures of the filter medium are limited to about 200 times or less as large as the average particle diameter of the raw material powder, as the excess aperture size whittles down the filtration effect. When the viscosity of the slurry is high so that the filtering process is not performed smoothly, the process is to be conducted in a few steps in which the aperture size of the filter medium is gradually reduced until a final stage where the target mesh aperture is achieved. Consequently, it is possible to reduce the incidence of defects attributed to a foreign matter as a cause and manufacture the ceramic of high quality with a high yield.