PURPOSE: To make easy the pass of the reactant gas, and to make the reaction product possible to be obtained at high yield, by installing in the apparatus the meshy or linear heaters horizontally into stages more than one.

CONSTITUTION: The cooling water is passed through the bell jar 3, the electrode 5 and the base body 16 so as to prevent the temperature rise at places except the reaction zone. The vacuum pump is actuated for evacuating the inside of the vessel, and the inert gas like as Ar, etc., or the reducing gas like as H2, etc., is introduced in from the gas inlet 1 for displacing the gas. The electric power is supplied from the electrode 5 to the heater 6 for heating; and when the temperature detected by the thermocouple 8 arrives at the specified temperature, the reactant gas is introduced in from the gas inlet 1. After the reaction comes to end, the heating of the heater 6 is stopped and the inert gas or the reducing gas is discharged from the vessel, and then, the unoxidized powder is taken away from the specimen reservoir 10. This apparatus is suitable for synthesizing such unoxidized powders as silicon nitride, silicon imide, silicon carbide, boron nitride, aluminum nitride, etc.