PURPOSE: To produce carbonaceous microballoons in improved yield, by feeding a powdery or granular thermoplastic organic material into an inert gas atmosphere in the form of phlasma generated by irradiation with microwaves.

CONSTITUTION: A wall 2 of a vessel 1 is cooled with a cooling medium 3, and the vessel 1 has a microwave plasma generator 4 mounted to the wall 1 and a discharge outlet 6 for an inert gas after use having a wire net 5 for preventing a scattering of carbonaceous microballoons. The inert gas is fed from an inert gas feeder 80 into the microwave plasma generator 4, which is operated to form a plasma flame 10 of the inert gas in the form of plasma generated by irradiation with the microwaves from a tip nozzle part 9. A powdery or granular thermoplastic organic material in a hopper 7 is fed to the tip nozzle part 9 by an inert gas stream from an inert gas feeder 8 at the same time. The powdery or granular material is foamed and carbonized in the plasma flame 10, dropped through a wire net 50 to the bottom and taken out by a discharge mechanism 11.