PURPOSE: To reduce the starting pressure of extrusion and decrease the operating cost by inserting a piston plate, which divides a container into the intensifying room on the stem side and the extruding room on the die side, and the case to store the piston plate into the container and forming the groove to be used for connecting pressure media in both rooms mentioned above at a specific period.

CONSTITUTION: Piston plate 9, which divides container 1 into intensifying room 12 on stem side 11 and extruding room 13 on die side 3, and storing case 8 in which piston plate 9 can be inserted with free sliding are equipped in container 1 of the extruding machine by hydrostatic pressure. When billet 7 is extruded for molding to the space between mandrel 6 and die 3 by working pressure plunger stem 11, intensifying room 12 and extruding room 13 are connected through groove 14 cut on the inner surface of storing case 8 and groove 15 of mandrel holder 5 when piston plate 9 is slided up to the fixed depth and pressure media in room 12 flows into room 13 and is replenished. Thus, the starting pressure for extrusion can be reduced by decreasing the quantity of pressure media and the temperature fall of billet 7.