The manufacture of structural elements for aircraft requires the use of complex and costly methods, particularly in the case of parts of elongate overall shape and variable thickness or cross section made of composite material. The disclosure herein proposes to overcome this problem by a method that allows the manufacture of a structural part from a preform made of composite material of simple shape obtained by pultrusion and of one or more reinforcing elements made of composite material and secured by cocuring with the preform to a region of this preform which region is to be reinforced.