To economically manufacture a duct capable of preventing unintended warm air and water from being sucked, and having a reducing function of intake noise.

After setting a parison between opened first molding die 20 and second molding die 22, the first molding die 20 and the second molding die 22 are closed, and compressed air is blown into the parison. The parison is expanded and pressed against the inside surfaces of a first recessed part 20a, a second recessed part 22a, a third recessed part 24 and a fourth recessed part 26, whereby a thin film part 18 extended outward from a duct body 14 in a cylindrical shape to close a pipe part 16 communicating with an air passage 14a at one end, and an extended end of the pipe part 16 is molded integrally with the duct body 14 with the air passage 14a formed therein. By plunging the thin film part 18 inward, an intake duct having the thin film part 18 adjacent to the duct body 14 is provided.