PURPOSE: To carry out high temp., high press. treatment of a product in a simple apparatus by hermetically sealing an autoclave; placing the product and slid or liquid CO2 in the autoclave; and gasification expanding the CO2 by heating to simultaneously provide high temp. and high press.

CONSTITUTION: Cylindrical autoclave 1 are hermetically sealed with plugs 2,3 to form a high press. chamber. Heater 7 is set in the chamber through heat insulating layer 5. Container 7 housing product 4 to be treated and dry ice is placed in the chamber. By supplying current to heater 8, the dry ice is converted into gaseous CO2 to further raise the temp. Thus, product 4 is heated to high temp., and high press. is generated by expansion of the CO2 to carry out high temp., high press. treatment of product 4. Liquid CO2 may be used in place of dry ice. Autoclave 9 and dry ice container 11 may be connected with a pipe to heat dry ice with inner heater 12 or outer heater 16.