PURPOSE: To obtain the subject ceramics excellent in heat resistance, mechanical strength and machinability by mixing plural specific glass powders, then granulating the mixture, forming and heating the granulated mixture under prescribed conditions and providing a burned compact.

CONSTITUTION: A raw material mixed so as to provide a composition ratio of 11.0-11.5wt.% K2O, 23.5-24.8wt.% MgO, 55.0-56.8wt.% SiO2, 8.4-12.6wt.% F2 and 0.86-0.99wt.% ZrO2 is melted at 1400-1500°C heating temperature to prepare glass, which is then pulverized. With 30-95wt.% resultant glass powder, is mixed 5-70wt.% glass powder obtained by melting a raw material mixed so as to afford a composition ratio of 5.0-6.0wt.% H2O, 9.5-11.0wt.% MgO, 46.0-48.0wt.% SiO2, 21.0-23.0wt.% CaO, 0.2-0.5wt.% CaF2, 5.0-6.5wt.% MgF2 and 8.0-8.5wt.% P2O5 at 1400-1500°C heating temperature to provide glass powder and subsequently pulverizing the aforementioned glass powder. The obtained mixed powder is then granulated and the granulated mixture is filled in a metallic mold and formed under ≥200kg/cm2 pressure to afford a compact, which is heated at 1050-1150°C to provide a burned compact.