To provide an air volume-adjusting device in which resistances against airflow under a full-opened state can be reduced and either noise or vibration as well as its loss can be restricted.

A vane base 12 for supporting a rotary vane 13 is arranged at an airflow-feeding pipe 11 in such a way that the vane can be turned by a predetermined angle. The rotary vane 13 is turned to adjust an opening area within the airflow-feeding pipe where the airflow can pass so as to adjust an air volume and further under a full-opened state, the vane base 12 and the rotary vane 13 are set to be in parallel with an axial direction of the airflow- feeding pipe. With such an arrangement as above, it becomes possible to keep a more opening area within the airflow feeding pipe 11 under the full-opened state, thereby reducing resistances against the airflow, a stable state can be assured against an air pressure to cause a pressure loss to be hardly generated and noise can also be reduced. In addition, the airflow is not biased within the airflow-feeding pipe, a biased airflow or the like is not generated and no bad influence is applied to its downstream side.