PURPOSE: To directly bring a rubber mold to pressure forming by a hydrostatic press machine by forming a flange on the outside of both end parts of a drum mold made of rubber for prescribing an external shape of a powder forming material filled in the inside, and the outside of the fitting part of the upper and lower dies fitted to them, respectively.

CONSTITUTION: A formed part of a drum mold 3 to which the lower die 2 is fitted is filled with a powder forming material 4. Subsequently, the upper die 1 is fitted to the drum mold 3 to which filling is ended, and the outside periphery of a flange part is sealed with a tape 5. Next, this rubber mold 3 is put into a pressure medium of a pressure container of a hydrostatic press machine and pressurized. By this pressurization, the rubber mold 3 is brought to elastic deformation to the inside and the powder forming material 4 is pressed and solidified. A flange 3a of the rubber mold 3 and each flange 1a, 2a of the upper die 1 and the lower die 2 come into contact strongly and closely with each other, and it does not occur that a pressure medium intrudes into the formed part of the rubber mold. In such a way, the process in which the rubber mold 3 is put into a bag made of thin rubber and brought to vacuum deaeration becomes unnecessary, and the work efficiency can be improved.