A large amount of spinel type silicon nitride powders of a high-pressure phase is produced by mixing raw powders having a particle size of 10 m or less of -, - or amorphous silicon nitride, each of which is a low pressure phase, with non-nitrified metal powders at a ratio of 50 weight % or more, preferably copper powders with a particle size of 100 m or less, forming a compact with porosity of 50% or less and 10% or more, preferably around 30%, by pressing mixture powders obtained, and subjecting the compact to shock wave compression treatment at a pressure of 20 GPa or more, preferably around 60 GPa.