To provide a driving circuit for preventing internal contamination of a handpiece, preventing patient's saliva and cutting pieces of a tooth from being sucked into the handpiece when stopping to use the handpiece.

A proportional control valve 5 is provided on an air supply pipe 4 for supplying compressed air from a compressed air source 3 to an air turbine 2; an exhaust valve 7 is provided on an air discharge pipe 6; and when stopping to use the air turbine, and if the exhaust valve 7 is closed, the air supply flow of the proportional control valve 5 is simultaneously adjusted not to rotate the air turbine by electric control from a control circuit 8, whereby the interior of the handpiece is always kept at the positive pressure state.