An oxidation-resistant silicon nitride material possessing a surface layer of a phase of closely packed crystals is produced by a procedure which comprises covering the surface of a silicon nitride substrate with a layer of an alkali metal compound, firing the coated substrate at a temperature of between 800.degree. C. and 1,300.degree. C., thereby forming an alkali metal-containing vitreous coating layer on the surface of the silicon nitride substrate, and thereafter removing the coating layer.