A silicon nitride sintered product comprising silicon nitride grains and a grain boundary phase, wherein the grain boundary phase consists essentially of a single phase of a Lu4Si2O7N2 crystal phase, and the composition of the silicon nitride sintered product is a composition in or around a triangle ABC having point A: Si3N4, point B: 28 mol % SiO2-72 mol % Lu2O3 and point C: 16 mol % SiO2-84 mol % Lu2O3, as three apexes, in a ternary system phase diagram of a Si3N4—SiO2—Lu2O3 system. Also disclosed is a silicon nitride sintered product comprising silicon nitride grains and a grain boundary phase of an oxynitride, wherein the composition of the sintered product is a composition in a triangle having point A: Si3N4, point B: 40 mol % SiO2-60 mol % Lu2O3 and point C: 60 mol % SiO2-40 mol % Lu2O3, as three apexes, in a ternary system phase diagram of a Si3N4—SiO2—Lu2O3 system.