The invention concerns a supply air terminal device (10) including side plates (12) and an air guiding part (13). A heat exchanger (14) is fitted in the device below a supply air chamber (11) for supply air in between air guiding parts (13) located on both sides of the central axis (Y₁) of the device. In the device, the supply air chamber (11) includes nozzle apertures (12a₁, 12a₂, . . . , 12b₁, 12b₂ . . . ) to conduct fresh supply air into a side chamber (B₁) and to induce a flow of circulated air (L₂) from the room space through the heat exchanger (14) into the side chamber (B₁). Using the heat exchanger (14) the circulated air may be either cooled or heated. The equipment includes a control device (15) for the induction ratio of the supply air flow (L₁) and the circulated air flow (L₂) for controlling in which ratio there is fresh air (L₁) and circulated air (L₂) in the combined air flow (L₁+L₂).