To absorb a mounting error even when an mounting error is produced on a stiffener by providing an extension width which extends one side of two duct boards which adjoin each other and joining each adjacent stiffener by way of the extension width.

This duct 1 consists of duct boards 2 and 3 and is reinforced with stiffeners 4 and 5 which wind up around the duct 1. The joining areas of the duct boards 2 and 3 are completely sealed by seal welding. The boards are welded over the whole length or partially so as to hold proper strength. In addition to that, each joining area of the adjacent boards 2 and 3 is welded by extending one duct board 2 so as to provide an extension width 13. Both stiffeners 4 and 5 are joined with each other by way of this extension width 13. The presence of this extension width 13 makes it possible to absorb pitch errors of the stiffeners 4 and 5 since the adjacent stiffeners 4 and 5 join each other by way of this extension width 1 even if the stiffeners 3 and 4 produce any pitch errors.