An aircraft turbojet engine fan casing in the form of a box section is provided by the present disclosure. The box section includes a radially interior wall of which is able to form an internal skin of a cold air flow duct of a nacelle in which said turbojet engine is intended to be mounted, and a radially exterior wall of which is able to form an external skin of said nacelle, the box forming a module forming an entire thickness of the nacelle, and placed between an upstream portion of the nacelle, forming the air intake, and a downstream casing portion, on which a cascade edge of a thrust reverser can be fixed.