To improve rigidity of a lower portion of an air intake duct storing an interior-partitioning part in an upper portion.

An interior-partitioning part 10 is supported by an upper end of a fixing support 19 from below so as to be disposed below a molding machine head 20. A partitioning plate 24 is hanged on a halfway part of the fixing support 19 so as not to drop. A tubular molten resin 21 is injected downward from the molding machine head 20 for enveloping the interior-partitioning part 10 and the partitioning plate 24. The molten resin 21 enveloping the same is clamped from both sides with molds 22, 23, by blowing compressed air in this state from the fixing support 19, the molten resin 21 is swelled so as to force the same to the inner wall of the molds 22, 23 for cooling down and solidifying the molten resin 21 for forming a duct body. Thereafter, the molds 22, 23 are opened and the fixing support 19 is pulled off downward so as to have the interior-partitioning part 10 and the partitioning plate 24 remain in the duct body as stored parts.