To easily produce a high precision inside duct 5 in which a thin film part (especially a body part) is very thin.
A production method includes a process in which a resin sheet material 40 is heated and softened, a process in which a halved square tube body 41 is obtained by vacuum-forming, air pressure forming, or vacuum/air pressure forming the softened resin sheet 40, and a process in which the two halved square tube bodies 41 molded in the shape of a halved square tube are joined/fixed together to make their inner surfaces face each other to obtain the thin inside duct 5 in which the film thickness of the body part is as thin as 0.5 mm or below. By using the production method, a failure such as the break of the halved square tube body 41 after the vacuum forming, the pressure forming, or the vacuum/pressure forming can be prevented, and the film thickness in the duct periphery direction of the body part of the inside duct 5 can easily be made uniform, improving the dimensional accuracy in the wall thickness (film thickness) direction of the body part of the inside duct 5.
NAKAYAMA TOSHIAKI
NISHIO YOSHITAKA
Shinji Hase