An air terminal device for a ventilation system includes a pressure box, with an inlet admitting supply air into the pressure box and outlet openings for admitting supply air out. The air terminal device further includes a cover plate to control and change the open area of the outlet openings. The outlet openings are in a wall of the pressure box forming an outlet surface of the pressure box. The cover plates are arranged to make contact with and slide relative the outlet surface while changing the open area of the outlet openings. The cover plate is located on the high pressure side of the outlet surface. The cover plate cooperates with the outlet surface such that there is at least one outlet opening or suction opening being partly or fully covered by the cover plate also when the air terminal device is set to maximum flow.