PURPOSE: To obtain the titled high-purity and fine particulate AlN powder capable of readily providing a sintered material having a high density and thermal conductivity by sintering at a low temperature, by introducing metallic Al and NH3 gas into a mixed gas of N2/Ar which is converted into a plasma flame by high-frequency discharge.

CONSTITUTION: The interior of a high-frequency heating plasma apparatus for synthesizing fine particles is previously evacuated by an evacuating device 12. N2 gas is filled from a mixed gas inlet 8 and Ar gas is fed from a plasma gas feed port 4. A high-frequency electric power, e.g. 20KW, is applied at the same time to generate a plasma flame 5 through a quartz plasma generation tube 2 can high-frequency coil 1. H2 gas and mixed gas of NH3/H2 are respectively introduced from the respective gas feed ports 6 and 8 and metallic Al powder is introduced from a raw material feed port 14 to initiate nitriding reaction and collect the resultant titled AlN powder having ≤0.1μ particle diameter of primary particles with a powder collecting device 10. The obtained powder is then sintered in a vacuum or nonoxidizing atmosphere under ordinary pressure without adding a sintering assistant and afford a sintered material having ≥90% relative density and ≥60W/m.k thermal conductivity.