A stringer made of composite material for reinforcing aircraft skin panels. Plies are introduced at 90° in a segment close to the stringer run-out, and progressively reducing the number of plies at 0°, such that the majority of the number of plies is at 90° in an segment adjacent to the run-out, so the stiffness of the run-out is reduced, and the load it supports is also therefore reduced. This is an alternative solution to the solutions already existing for getting the stringer run-outs to support a smaller load, thereby reducing both the risk of the plies of the stringer peeling off and the risk of separation between stringer and skin panel. A method of manufacturing said stringer is also provided.