PURPOSE: To obtain products of uniform and high quality by passing gas sent by fans at gas suction ports through both outer ventilation paths which are divided and are in the opposite direction to each other, causing the gas to meet each other at a gas blowing port and to flow in an inner ventilation path and properly dividing the gas among gas suction ports to circulate it.

CONSTITUTION: As for a method for circulating gas in an autoclave, outer ventilation paths 11a, 11b are formed between an inner wall 2a and an air duct wall 10 of an autoclave and an inner ventilation path 12 is formed inside the air duct wall 10; plural gas suction ports 13 are provided at constant intervals in series in the upper part of the air duct wall 10 and plural gas blowing ports 14 are provided in the lower part corresponding to the gas suction ports 13; fans 15 are arranged each in the plural gas suction ports 13; and the outer ventilation paths 11a, 11b are divided every adjacent fans 15 by partition members 16 in the longitudinal direction of a vessel A to form the outer ventilation paths 11a, 11b. And gas sent by each fan 15 is passed through the outer ventilation paths 11a, 11b which are divided and are in the opposite direction to each other and meets each other at the gas blowing ports 14 to flow in the inner ventilation path 12 and the gas is properly divided among plural gas suction ports 13 to be circulated.