To provide a duct capable of achieving efficient circulation of temperature-conditioned air and the like and reduction of noise generated in driving and circulation associated with distribution/suction of the air, while saving a space.

In this duct in which an approximately linear first circulating passage 12 and an approximately linear second flow channel 14 connected with a driving device BW distributing/sucking a gas A at its one end portion, are intersected to form the bent portion 18, the bent portion 18 comprises a reflection wall 18a receiving the noise from the driving device BW propagating through a second circulating passage 14, and reflecting the same, a circulating member 20 having a ventilation volume within a range of 150-350 cm3/(cm2 S) is disposed in the R-shape along the circulation of the gas A at its corner portion 19, and a spatial portion 26 is defined between the reflection wall 18a and the circulating member 20.