Abstract: In the casing of a turbo-molecular pump there is a suction port at the suction side, and a discharge port at the discharge side. A stator is arranged in the casing and a rotor is journaled therein in magnetic bearings. The rotor is provided with rotor discs which are axially arranged along the longitudinal axis of the rotor, each of the rotor discs having vanes. The axial thickness and the outer diameter of each of the discs of the rotor are dimensioned so that the moment of inertia of the rotor about its axis of rotation exceeds its moment of inertia about a transverse axis through its center of gravity. Additionally, the vanes of the rotor disc adjacent the suction side of the pump are long and thin relative to the vanes of the other rotor discs of the rotor, and the diameter and thickness of the rotor disc adjacent the suction side of the pump is large relative to the diameter and thickness of the other rotor discs of the rotor.