To swiftly and homogeneously increase the temp. of the whole body in a furnace and to increase the working efficiency of the furnace by driving a fan in the meanwhile till the temp. reaches the one at which the degreasing of the object to be treated starts to progress and heating the object to be treated by a convection current.

A space S1 in a furnace and a heating space S2 are evacuated, and the heating of the object to be treated is started by a heater 4. While the vacuum in the furnace is controlled, the heating is executed, when the temp. of the heating space S2 reaches about 500°C, a binder is evaporated from the object to be treated, and degreasing starts. When the degreasing is finished, the temp. of the heating space S2 is raised to about 1,350°C, and sintering is executed, and, after the completion of the sintering, a cooling gas is introduced therein, a fan 32 is operated, and forced cooling is executed. At this time, in the process in which the object to be treated is heated, in the meanwhile till the temp. reaches the one at which the degreasing starts to progress, the fan 32 is driven, gaseous nitrogen is introduced, and the object to be treated is subjected to convection heating. When the temp. reaches about 150°C at which the evaporation of the binder is not expected, the driving of the fan 32 is stopped, the vacuum is raised, and the condition is returned to the one needed for the original degreasing stage.