We have developed a silicon nitride ceramic material which displays significantly higher resistance to thermal shocks and strength at high temperatures than conventional silicon nitride ceramics. Using silicon nitride (Si₃N₄) as a base, the material was developed by forming a grain boundary phase with high thermal conductivity and dispersing almost amorphous nanometer-order particles of boron nitride (BN) in the grain boundary phase. While the strength of conventional silicon nitride ceramics declines at a temperature difference of 1,000 °C, there was almost no deterioration in the strength of the developed material even when heated to 1,400 °C in an electric furnace and dropped into water ten times. In addition, the material displayed the same level of strength in high-temperature bending tests conducted in air at 1,200 °C as at room temperature.