To enhance degrease sintering performance effectively with no possibility of internal contamination of furnace or explosion.

A degrease atmosphere is limited to a processing space S1 in an inner box 2 disposed in a furnace 1 and the air Y is introduced directly into the processing space S1. A material to be processed is then degreased under a low pressure oxygen atmosphere while discharging the air Y directly to the outside of the furnace 1 and a transition is made to a sintering process by raising the temperature in the furnace 1 continuously. Consequently, degrease and sintering processed can be carried out continuously without causing any internal contamination of the furnace due to a binder and degrease process can be progressed efficiently when it is combined with oxidizing decomposition due to the air Y and low pressure atmosphere setting.