Abstract: A supply air device for spreading an air flow from a generally flat, perforated external element (21,22,23) comprising a box-like casing (10) serving as an air distribution chamber and having means for connection to a supply air duct, and at least one primary side-spreading device (18,26) located in the distribution chamber and being adapted to bring about a forwardly directed, transversally diverging air flow towards and through perforated external element. Side guiding plates (24,25) are provided in the distribution chamber so as to extend between the primary side-spreading device (26) and side edge portions (22,23) of perforated external element, guiding plates being bent into a convexly curved configuration, as seen from the front of the device, whereby the air flow (P1,P2,P3) is caused to additionally diverge transversally, especially to about 180.degree..