Attachment device and system for a vehicle roof box

Device and attachment system that can be used for fixing an object (1) such as a roof box onto a roof bar (4) of a motor vehicle. The device comprises a base (6) and a bracket (7) framing the bar (4) and a part of the object (1) to ensure their clamping. The bracket has a horizontal part secured at one of its ends to a vertical part connected with the base (6). Application to the fixing of accessories for motor vehicles by means of roof bars.