This device includes at least one peripheral wall (2, 6, 8) forming an air duct (4) and at least one first fin (10) rotatably mobile around a substantially longitudinal axis (A), at least one second fin (12) rotatably mobile around a substantially transverse axis (B), and at least one deflector (18) extending upstream of the second fin (12) and being rotatably mobile around a substantially transverse axis (C), the downstream end portion (20) of the deflector (18) being fastened to the upstream end portion (22) of the second fin (12) via a hinge (24). The axis of rotation (C) of the deflector (18) is mounted in at least one slot (28) of the peripheral wall (2, 6, 8), the slot (26) extending along a path at least partially not parallel to the upstream-downstream direction of the air duct (4).