PURPOSE: To provide a silicon nitride sintered compact low in mechanical strength drop-off when exposed to an atmosphere with its temperature ranging from room temperature to high temperatures (esp. up to 1500°C), having high resistance to oxidation.

CONSTITUTION: A compact of composition comprising (A) silicon nitride, (B) an oxide of group 3a element and (C) silicon dioxide with the molar ratio SiO2/RE2O3 of ≥2.0 is sintered in a nonoxidative atmosphere, and the product is temporarily held at a temperature falling between the temperature (Tm/2) 0.5 times the melting point (in terms of absolute temperature) of the glass produced at the grain boundary of the resultant sintered compact and the phase transition temperature Tt at which RE2Si2O7 (RE is group 3a element) crystal is converted from γ-type into β-type, and then further held at a temperature falling between Tt and the melting point of the grain boundary phase to deposit Si2N2O and/or RE2Si2O7 crystal phase as crystals each ≤0.3μm in mean grain diameter at the grain boundary of the silicon nitride crystal, thus obtaining the objective sintered compact.