PURPOSE: To enable the production of a sintered body having both uniform compsns, for its surface and internal structure, in the stage of producing a sintered hard alloy contg. carbonitrides of metals as a hard phase by annealing the alloy continuously or intermittently in a microwave plasma nitrogen atmosphere.

CONSTITUTION: The hard phase particles consisting of the metallic carbonitride of the compsn. expressed by the formula[I]are mixed with Co as a binder and after the mixture is pressurized and molded, the molding is sintered, whereby a sintered hard alloy is produced. Here, in the stage of putting moldings 5 into a reaction tube 4, and evacuating the inside of the tube 4 by suction with a pump 7, then subjecting the moldings to a sintering treatment for 1 hour at 1400°C with heaters 2, gaseous nitrogen from a nitrogen bomb 15 is supplied through an introduction pipe 8 for gaseous N2 into the tube 4 and at this time, plasma 9 is generated by the electric source from a microwave oscillator 12 and a tuner 11, and the gaseous nitrogen is supplied as gaseous plasma nitrogen into the tube 4. The removal of nitrogen from the surfaces of the moldings 5 is obviated and the superhard sintered body consisting of the metallic carbonitride having a uniform surface and uniform internal structure is obtained.