Lighting and/or signalling device of an automobile

The device (1) has a mounting unit (5) mounting a casing (B) on a chassis (C) of a motor vehicle. A fixation element (8) i.e. screw, fixes the mounting unit on the casing by partial insertion of the fixation element in a blind cavity (6) of the mounting unit. The casing comprises an orifice (10) for allowing a passage of the fixation element. A sealing joint (11) seals the orifice of the casing. The casing comprises an inner wall (12) provided with a circular compartment defining by an annular rib to partially receive the sealing joint.