PURPOSE: To prevent contamination in a hospital in dental treatment by eliminating the infiltration of blood, salver or the like from other patients into a handpiece.

CONSTITUTION: Upper and lower air chambers 10 and 11 are formed between bearings 5 and 6 fixing upper and lower shafts of a drive section 3 free to turn and upper and lover lid members 12 and 13 and air is supplied to the air chambers 10 and 11 so that an internal pressure in a head tip 2 is raised while a straightening means is provided in an air discharge port provided in the lid members 12 and 13 so that no disturbance is caused in the air discharged through air exhaust ports 14 and 15 from the air chambers 10 and 11. Moreover, the air is straightened by the straightening means to be discharged and hence, there is no Karman's vortex generated inside and near the air exhaust ports 14 and 15. This gets rid of suction phenomenon of outside air thereby eliminating contamination in the hand piece 1.