PURPOSE: To perform rapid and effective cooling of the interior of a furnace chamber surrounded with the heat insulation layer of a hot isotropic pressure device.

CONSTITUTION: During the starting of cooling, pressure medium gas is circulated through a high pressure vessel 1 by a flow circulated through the inside and the outside of a furnace chamber 5 and a flow flowing through the furnace chamber 5. After temperature in the high pressure vessel 1 is lowered, a pressure medium gas is circulated in the high pressure vessel 1 only by a flow circulating through the inside and the outside of the furnace chamber 5. In this way, during the starting of cooling, the interior of the furnace chamber is rapidly and uniformly cooled by a circulation flow through the inside and the outside of the furnace chamber through natural convection owing to a density difference of the pressure medium gas and a circulation flow in the furnace chamber through a pressure medium gas forced circulation mechanism. After temperature is lowered, the pressure medium gas is brought into efficient contact with the inner wall surface of the high pressure vessel through circulation of the pressure medium gas through the inside and the outside of the furnace chamber. A density difference of the pressure medium gas is eliminated through reduction of a temperature difference between the inside and the outside of the furnace chamber and even after the inside and the outside of the furnace chamber are brought into an approximate balancing state, rapid and high-efficient cooling can take place.