Abstract: This device (30) is used to stiffen a box part (10) for receiving a motor-drive device for operating a blind, the box part having a U section with an external surface (13) and an internal surface (14). This device is independent of the supporting means for the box part relative to a support structure and comprises a first contact element (31) intended to cooperate with at least a part of an external surface (13) of the box part; a second contact element (32) intended to cooperate with at least a part of an internal surface (14) of the box part, and tightening means (35) acting on the first contact element (31) and/or the second contact element (32), in a direction of convergence of this element and of the part of the surface of the box part (13, 14) with which it is intended to cooperate. A box part (10) equipped with such a device is stiffened in an axis (Y-Y?) perpendicular to its longitudinal axis (X-X?).