A casing including a wall defining an internal housing which allows retraction of a corresponding landing gear into the interior of the housing. The wall separates the non-pressurized internal housing from a surrounding pressurized zone of the aircraft. The separation wall is a shell which envelops the shape of the landing gear, on the side opposite the opening of the internal housing, and the reinforcement structure includes on one hand longitudinal reinforcement elements extending on both sides of the shell and on the other hand transverse reinforcement elements connecting the longitudinal reinforcement elements. The shell has a first zone of considerable width accommodating the wheels of the landing gear, a second zone of lesser width accommodating the rod of the landing gear, and a third zone running from the second zone of lesser width, becoming wider and accommodating a portion of the mechanism for deployment/retraction of the landing gear.