Abstract: A centering device in a machine tool having a chuck for clamping a work includes a clamp device for clamping and centering the work at a different position with respect to the chuck. The clamp device has an actuator driven by a pressure medium and a holder operable by the actuator for clamping the work with a pressure corresponding to the pressure of the pressure medium and for releasing the work. A control device is provided for controlling the pressure of the pressure medium to a value which is appropriate to center the work by the clamp device and which is previously determined according to a diameter of the work to be clamped.