A device for fastening a downstream end of an air inlet on a fan casing of an aircraft turbojet engine nacelle is provided. The device includes an acoustic shroud, a first flange coupled to the fan casing, a second L-shaped flange having a base coupled to the acoustic shroud, and a collar linked to the first flange. The acoustic shroud extends from an upstream segment to a downstream segment which defines an inner skin and an outer skin that encloses an acoustic panel. The collar further includes a protrusion that extends from the outer skin to the downstream segment.