Silicon nitride sintered bodies are disclosed which contain silicon carbide therein and in which intergranular phases between silicon nitride particles are substantially crystallized. Further, a manufacturing method of the sintered bodies is disclosed, in which a silicon carbide powdery raw material is used as an additive when preparing raw powders and the intergranular phases are crystallized during a temperature descending stage following a firing. Silicone carbide effectivley promotes densification of the structure of the sintered body and crystallization of the intergranular phases, thereby making it possible to provide the sintered bodies having intergranular phases with little glass phases uncrystallized and excellent high-temperature strengths.