A positioning device for assembling parts on a structure is disclosed. More specifically, the positioning device is designed for assembling fixed brackets to an aircraft structure. A laser projector is fastened directly on the rigid aircraft structure; its position is therefore stable in relation to the latter, even if the structure undergoes slight deformations. The position of the projector is determined accurately by utilizing targets, which are also fastened to the primary structure. A method for implementing the positioning device is also disclosed.