PURPOSE: To facilitate the control of the amt. of oxygen in a sintered body and to obtain a homogeneous sintered body having a desired compsn. of the grain boundary phase and high strength at high temp. by sintering a molded body of silicon nitride contg. a sintering aid and subjecting the resulting primary sintered body to hot isostatic pressing(HIP) in a capsule.

CONSTITUTION: When silicon nitride contg. a sintering aid is molded and sintered to produce a sintered body, the molded body is primarily sintered at 1,700-1,900°C in an atmosphere of nitrogen under 5-10kg/cm2 pressure and the resulting primary sintered body having ≤10% open porosity is sealed in a capsule and subjected to HIP.