To provide a structure for connecting a first duct formed of a foaming resin sheet under vacuum in tight contact with a second duct with rigidity.

In the structure, the first air conditioning duct 12 formed by connecting both the end edge parts of the half duct body formed of the foaming resin sheet under vacuum is connected to the second air conditioning duct 14 with rigidity. The structure comprises an inwardly extending engaging piece 18 formed at the opening end of a first opening part 16 of the first duct intersecting a central axis L from the opening end, an expansion allowing part 22 formed from the first opening part to the engaging piece to expand the opening part outward and a groove part 20 slightly inwardly recessed on the periphery of the outer side surface from a second opening 24 of the second duct. Thus, the first opening part is opened outward by a prescribed dimension by the expansion allowing part to allow the opening side of the second duct to be internally inserted and the engaging piece formed in the first opening part to be fitted to and engaged with the groove part of the second opening part.