The present invention relates to a handpiece having a direct cooling type slimmed driving part, comprising: an outer housing provided in a rod shape so as to be graspable by the hand a cylindrical core inserted into the outer housing of which an inside is penetrated from a front portion to a rear portion such that a coil is wound on an inner peripheral surface thereof, and having an air hole and a water hole respectively penetrated from a front surface to a rear surface cylindrical pipes respectively inserted into the air hole and the water hole of the core such that both side end portions respectively protrude forward and backward, and penetrated for the passing of the air and the water a rotor inserted into the core, provided with a transmission shaft respectively extending forward and backward at one side and the other side, and made from a cylindrical magnetic body cylindrical support rings respectively fitted on the both sides of the transmission shaft of the rotor, each one surface in the rotor direction comes in contact with the rotor, and another side surface is formed to protrude a pair of bearings respectively provided to a front and rear of the support rings and insertively fitted to the transmission shaft so as to rotatively support the rotor a coupling provided to a front of the rotor and coupled to the transmission shaft a front housing coupled to a front of the outer housing and respectively formed with an air outlet and a water outlet by penetrating an outer peripheral surface thereof so as to discharge the air and the water supplied from the pipes, and of which an inside is penetrated from a front to a rear to insert the transmission shaft and the coupling a front cap provided to a rear of the core, having a forward open groove formed on the inside to allow the bearing to be inserted, and having a plurality of through holes penetrated from a front to a rear at a distance from the groove a rear cap provided to a rear of the front cap and having a plu
