Sintered silicon nitride-based body

A sintered silicon nitride-based body comprising 20% or less by weight of a grain boundary phase and the balance being a major phase of grains of silicon nitride and/or sialon, wherein the major phase contains a grain phase of a beta -Si3N4 phase and/or a beta '-sialon phase, and a quantitative ratio of the grain phase of the beta -Si3N4 phase and/or the beta '-sialon phase is in a range of 0.5 to 1.0 relative to the major phase; the grain boundary phase contains Re2Si2O7 (wherein Re represents a rare-earth element other than Er and Yb) as a first crystal component and at least one of ReSiNO2, Re3Al5O12, ReAlO3, and Si3N4.Y2O3 as a second crystal component; and a quantitative ratio of the first and second crystal components in the grain boundary phase to the grain phase of beta -Si3N4 phase and/or the beta '-sialon phase ranges from 0.03 to 1.6. The sintered body is produced by mixing a specific sintering aid and silicon nitride-based powder, sintering the mixture and heat treating the sintered body for nucleation and crystal growth within the temperature range of from 1050 DEG to 1550 DEG C.