Improved-structure instrument for allowing the alignment of fixing screws to be inserted in transverse holes (3, 13) of nails for long bones, in particular medullary nails, of the type comprising: - an arched arm having a free distal end connectable to a proximal end of a head of the nail in which said transverse holes (3, 13) are formed, and - a proximal handle portion as well as a connection portion between the distal end and the proximal handle portion, - at least one through-hole (23, 33) in said connection portion for receiving a guide tube for a drilling bit, said at least one through-hole (23, 33) having an axis coinciding with the axis of one of said transverse holes (3, 13); - locking means associated with said at least one through-hole (23, 33) so as to lock in a stable position the corresponding tube inserted in the instrument; wherein - at least a chamber is formed inside the body of said connection portion around said through-hole for housing said locking means; - at least a lateral opening in communication with said chamber; - a two components (9, 39) quick-locking cam assembly forming said locking means: a first component being integral with said body and placed around said guide tube and the other component including a cam and being angularly movable with respect to the first component; said other component being provided with a first hole through which the tube passes; said first component being provided with a second hole through which the tube passes; the first and second holes (22, 32) are slightly axially offset; said first hole having a flat portion protruding essentially along a chord of the circular shape of the first hole so as to form said cam; - a trigger element of said other component projecting from said lateral opening to be manually operated thus moving the other component with said cam interfering with said guide tube, a relative rotation of the components (39, 9) causes a constriction of the guide tube inserted into the holes (22, 32) by means of the flat portion and stable locking in position of the said guide tube. - an arched arm having a free distal end connectable to a proximal end of a head of the nail in which said transverse holes (3, 13) are formed, and - a proximal handle portion as well as a connection portion between the distal end and the proximal handle portion, - at least one through-hole (23, 33) in said connection portion for receiving a guide tube for a drilling bit, said at least one through-hole (23, 33) having an axis coinciding with the axis of one of said transverse holes (3, 13); - locking means associated with said at least one through-hole (23, 33) so as to lock in a stable position the corresponding tube inserted in the instrument; wherein - at least a chamber is formed inside the body of said connection portion around said through-hole for housing said locking means; - at least a lateral opening in communication with said chamber; - a two components (9, 39) quick-locking cam assembly forming said locking means: a first component being integral with said body and placed around said guide tube and the other component including a cam and being angularly movable with respect to the first component; said other component being provided with a first hole through which the tube passes; said first component being provided with a second hole through which the tube passes; the first and second holes (22, 32) are slightly axially offset; said first hole having a flat portion protruding essentially along a chord of the circular shape of the first hole so as to form said cam; - a trigger element of said other component projecting from said lateral opening to be manually operated thus moving the other component with said cam interfering with said guide tube, a relative rotation of the components (39, 9) causes a constriction of the guide tube inserted into the holes (22, 32) by means of the flat portion and stable locking in position of the said guide tube. - at least a chamber is formed inside the body of said connection portion around said through-hole for housing said locking means; - at least a lateral opening in communication with said chamber; - a two components (9, 39) quick-locking cam assembly forming said locking means: a first component being integral with said body and placed around said guide tube and the other component including a cam and being angularly movable with respect to the first component; said other component being provided with a first hole through which the tube passes; said first component being provided with a second hole through which the tube passes; the first and second holes (22, 32) are slightly axially offset; said first hole having a flat portion protruding essentially along a chord of the circular shape of the first hole so as to form said cam; - a trigger element of said other component projecting from said lateral opening to be manually operated thus moving the other component with said cam interfering with said guide tube, a relative rotation of the components (39, 9) causes a constriction of the guide tube inserted into the holes (22, 32) by means of the flat portion and stable locking in position of the said guide tube.