A dentists' preparation instrument (1) comprises a turbine (4) for driving a tool (3) using compressed air, and a rotor (6) that is arranged in a turbine compartment (5), rotates about a longitudinal axis (7), and has blades (11) extending to a face (13) of the rotor (6). The turbine compartment (5) has a discharge port (12) for the compressed air towards a returning air duct (8). Said discharge port (12) is disposed in such a way that both the face (13) of the rotor (6) and parts of the blades (11) sweep past the discharge port (12) as the rotor rotates about the longitudinal axis (7). For deceleration purposes, the rotor (6) has a deceleration contour (9) on the face (13), said deceleration contour being formed by projections (14), and the turbine compartment (5) has a deceleration contour chamber (15) inside which the deceleration contour (9) is accommodated. The discharge port (12) in part also extends across the deceleration contour chamber (15).