To provide a gas circulation method and device for an autoclave, which further improves heat propagation efficiency with respect to a forming material, and achieves power-saving operation.

In the hot air circulation method for an autoclave, a muffle furnace 3 is installed in a pressure vessel 2, a heat is supplied to gas circulating in the muffle furnace 3, and then, a material is pressurized, heated, and cooled, and the forming material is bonded and formed in the pressure vessel 2. In the hot air circulation method and its device, a spiral flow generating a mixing turbulent flow is formed in a main flow of gas along a longitudinal axis in the muffle furnace 3.