An aligning device (1) for correctly inserting a screw into a respective through hole (10) of an endomedullary prosthesis (100) implanted in a bone tissue, comprising support means (3) associated with said prosthesis (100) and carrying guide means (2) defining an intervention aiming line (X-X) for perforation means for said bone tissue, said guide means (2) being detectable by selective detection means, and further comprising orientation means (5, 8) for spatially orientating said support means (3), said support means (3) further carrying a target (4) with sight (4') detectable by said detection means and positioned aligned with said aiming line (X-X), said hole (10) being interposed between said guide means and said target, with its axis disposed on said aiming line (X-X).