A dental hand-piece including mechanical organs comprising a tool holder assembly for fastening and driving in rotation a dental instrument about a driving axis and a chain of organs for the mechanical transmission of the rotational motion, said mechanical organs being mounted within a body comprising a head and a handle, a) the body being formed of one single electrically insulating part or casing one portion of which serves as a handle and the other portion forms a head, the head including a first recess ending with at least one opening dimensioned so as to permit the insertion of the organs of the head and their assembling inside the latter, the handle including a second longitudinal recess with a rectilinear axis ending, on the one hand, at the end of the handle with an opening and, on the other hand, into the first recess with a side opening, the opening being dimensioned so as to permit the insertion of the internal organs of the handle and their assembling inside the latter, b) in the handle, the electric current being conducted from a socket for connection with a motor by the chain of organs for mechanical transmission of the rotational motion formed as follows: - in the handle, the electrical current passes from a socket to a fixed outer ring of a first bearing, and to a first spring, then to a fixed outer ring of a second bearing, then to a ring axially stopped by a first shoulder in the casing, then to a spring axially stopped by a second shoulder of the casing, the first and second bearings (12, 14) carrying a shaft for transmission along the longitudinal axis of the handle, or first axis of the hand-piece and the springs (13 and 16) being compression springs the spires of which are arranged externally to the transmission shaft, - in the head of the counter-angle, with its second axis or driving axis carrying two ball bearings, namely an upper bearing the outer ring of which cooperates with the second spring of the handle and a lower bearing the axial backlash of which is taken up by means of a springy washer, c) in the head, a barrel-shaped pinion mounted on the driving axis integral with the inner rings of the bearings including teeth meshing with teeth of an output pinion of the handle, the barrel-shaped pinion being conductive and ensuring the electrical connection to the instrument and the mechanical driving of the instrument. a) the body being formed of one single electrically insulating part or casing one portion of which serves as a handle and the other portion forms a head, the head including a first recess ending with at least one opening dimensioned so as to permit the insertion of the organs of the head and their assembling inside the latter, the handle including a second longitudinal recess with a rectilinear axis ending, on the one hand, at the end of the handle with an opening and, on the other hand, into the first recess with a side opening, the opening being dimensioned so as to permit the insertion of the internal organs of the handle and their assembling inside the latter, b) in the handle, the electric current being conducted from a socket for connection with a motor by the chain of organs for mechanical transmission of the rotational motion formed as follows: - in the handle, the electrical current passes from a socket to a fixed outer ring of a first bearing, and to a first spring, then to a fixed outer ring of a second bearing, then to a ring axially stopped by a first shoulder in the casing, then to a spring axially stopped by a second shoulder of the casing, the first and second bearings (12, 14) carrying a shaft for transmission along the longitudinal axis of the handle, or first axis of the hand-piece and the springs (13 and 16) being compression springs the spires of which are arranged externally to the transmission shaft, - in the head of the counter-angle, with its second axis or driving axis carrying two ball bearings, namely an upper bearing the outer ring of which cooperates with the second spring of the handle and a lower bearing the axial backlash of which is taken up by means of a springy washer, - in the handle, the electrical current passes from a socket to a fixed outer ring of a first bearing, and to a first spring, then to a fixed outer ring of a second bearing, then to a ring axially stopped by a first shoulder in the casing, then to a spring axially stopped by a second shoulder of the casing, the first and second bearings (12, 14) carrying a shaft for transmission along the longitudinal axis of the handle, or first axis of the hand-piece and the springs (13 and 16) being compression springs the spires of which are arranged externally to the transmission shaft, - in the head of the counter-angle, with its second axis or driving axis carrying two ball bearings, namely an upper bearing the outer ring of which cooperates with the second spring of the handle and a lower bearing the axial backlash of which is taken up by means of a springy washer, c) in the head, a barrel-shaped pinion mounted on the driving axis integral with the inner rings of the bearings including teeth meshing with teeth of an output pinion of the handle, the barrel-shaped pinion being conductive and ensuring the electrical connection to the instrument and the mechanical driving of the instrument.