PURPOSE: To remove the residual carbon from the injection-molded body of a mixture of the raw material and wax after the body is degreased and before the body is sintered.

CONSTITUTION: A mixture of the raw material and wax is injection-molded, the molded body is degreased, hydrogen gas H2 kept at a temp. where the H2 reacts with carbon C is supplied into a tight box 4 under the first pressure P1, methane gas CH4 generated by the reaction is removed from the box 4 under the low second pressure P2, and the process is repeated to decarbonize the molded body.