PURPOSE: To suppress the variance of the shape and volume ratio of a fibrous preformed body by feeding the water having the volume corresponding to the objective fibrous/air ratio into a pressurizing chamber at the time of pressing the material for the preformed body.

CONSTITUTION: A mold 2 contg. the material 6 for hte preformed body is set in a rubber cylinder 13 while the rubber cylinder is expanded into the large diameter, and the mold 2 is inserted between a cover plate 12 and a bottom plate 11. A vacuum pump is operated to give absorption workng into the mold 2. A piston 21 of an oil hydraulic cylinder 19 is sleeved in such a manner that the volume of a primary oil hydraulic chamber C1 is shrunk and a piston 18 of a water pump 16 is sleeved in such a manner that a water contg. chamber B is shrunk to feed the water in the water contg. chamber B into the pressurizing chamber A. The material 6 for the preformed body is pressed via the rubber cylinder 13 into the mold 2 by the fed water, by which the fibrous preformed body 1 is molded and the fibrous volume ratio thereof is decided. Since water acts the pressure medium, the durability of the rubber cylinder 13 can be improved.