PURPOSE: To eliminate the temp. difference between the recessed part or shaded part of a formed article of intricate shape and the other parts and to obtain a high-quality formed article by circulating a gas through an outer passage and a duct properly backward and forward.

CONSTITUTION: The pressure vessel A of an autoclave is provided with a door 2a, a gas duct 6a, an outer gas passage 6b formed between the vessel wall and gas duct wall and an axial fan 22. A forming material 1 is placed in the vessel which is then closed by the door and pressurized, heated and formed. In this case, a temp. control means H is provided, the control means is operated to drive the fan 22 properly forward or backward, and the gas is circulated forward or backward through the passage 6b and duct 6a.