An air supply shutter for ventilating closed buildings includes a marginal frame with interconnected upper, lower and lateral frame members to define a centrally disposed air vent opening having inlet and outlet sides. A shutter flap is pivotally mounted on the frame for rotation about a generally horizontal axis between closed and open positions. A coupler is connected with the shutter flap and configured for engagement with an activator for shifting the shutter flap between the closed and open positions. The upper frame member includes an air guiding wall which is disposed on the outlet side of the air vent opening, faces the air vent opening, and extends upwardly at a predetermined angle to deflect the air flowing through the air vent opening.