PURPOSE: To produce a dense silicon nitride part by drying and sintering an extrudate of a mixture of silicon nitride powder and an aqueous solution of a prescribed ratio of mixture of polyacrylic acid-containing polymer, glycerin and oleic acid or the like.

CONSTITUTION: An aqueous solution is prepared by mixing by parts of weight of 0.1-0.65 of polyacrylic acid containing polymer, 0.5-1 of oleic acid, 0.1-2 of glycerin and 14-18.1 of water. Furthermore, silicon nitride powder and methyl cellulose are mixed. The powder mixture, the aqueous solution and a sintering aid agent are mixed to prepare an extrudate mixture with a pH of 2.5-3.5. Then the extrudate mixture is extruded and shaped, dried and sintered to form the dense silicon nitride material.