To provide a lightweight air duct with a sound absorbing material having the simple structure and easily installable in the air duct.

In this air duct with the sound absorbing material with the structure that the sound absorbing material is inserted into an enlarged diameter part 30 provided over the predetermined length at a part close to a central part in the axial direction of the air duct 21, the air duct 21 is divided into plural members at least at the enlarged diameter part 30. This division can be performed in a plane surface including the axis, or in a plane surface at a right angle against the axis. The sound absorbing material 40 and 40a desirably have the inner diameter larger than the inner diameter of the air duct 21 at both ends thereof.