FIG. 4 shows a part of an air-driven cutting device or handpiece for dental treatment, generally indicated by reference numeral (100). The handpiece (100) has a shaft section (102) and a head portion (104) which is formed integrally with the distal end of the shaft section (102). In the head portion (104), a housing (106) accommodates a rotor (110) for removably holding one end of a cylindrical cutting tool (108), bearings (112, 114) for rotatably supporting the rotor (110) and a turbine blade (116) mounted around the rotor (110) so that the turbine blade (116), the rotor (110) and the cutting tool (108) held by the rotor (110) are rotated by applying a compressed air from an air-supply passage (118) in the shaft (102), onto the turbine blade (116).
In this handpiece (100), an exhaust passage (not shown) is generally formed in the shaft section (102), and the air which has been applied onto the turbine blade (116) is exhausted through the exhaust passage. However, a part of the compressed air, after applying against the turbine blade (116), passes through the bearing (114) and a space (120) between the housing (106) and the rotor (110), and then, is injected at a high speed onto the distal end portion of the cutting tool (108) along the cutting tool (108). Further, the head portion (104) of the handpiece (100) has a mechanism for injecting cooling water to remove heat generated by the frictional contact between the rotating tool (108) and the tooth (122). This results in that the compressed air injected toward the distal end of the cutting tool (108) blows off the cooling water, so that a sufficient cooling effect can not be achieved.