PURPOSE: To efficiently execute vacuum sintering and HIP treatment with one device by axially movably inserting an inside cap into an outside cap having a large-diameter vacuum discharge port communicating with the inside of a high-pressure chamber so that said port can be freely communicated and shut off.

CONSTITUTION: An object to be treated 7 is loaded into a hot hydrostatic pressing device constituted by providing a heat insulating layer 4 and a heater 5 into a high- pressure chamber consisting of a high-pressure chamber 1 and delineated by an upper cap 2 consisting of the outside cap 2a and inside cap 2b and a lower cap 3 and is imposed on a hearth 6. The above-mentioned cap is then raised to communicate the vacuum discharge port 8 provided to the outside cap 2a with the high-pressure chamber and the inside of the chamber is evacuated to a vacuum and is heated by the heater 5 by which the object is vacuum-sintered. The cap 2b is thereafter pushed and moved in the axial direction from the outermost position 11 to the innermost position 12 to shut off the port 8. The caps 2, 3 are held by a press frame 10 and a pressurized gaseous pressure medium is introduced through a pressure port 9 into the high-pressure chamber and is heated by the heater 5, by which the object is isotropically subjected to an HIP treatment.