A diffuser unit for air conditioning systems includes an air supply duct having an air inlet and two air outlets, two dampers for adjusting the open area of the respective air outlets, and a linkage mechanism interconnecting the dampers and a drive unit. The dampers include arcuate end portions for contacting an inner wall of the duct and for preventing the generation of wind noise from air being supplied through the duck. The linkage mechanism is so constructed as to displace a selected one of the dampers while holding the other damper in its closed position.