This paper describes a vibrationless access control which is robust for parameter variations. It is important for the vibrationless access control of a positioning mechanism with mechanical flexibility to eliminate residual vibration at a target position even if natural frequencies of the mechanism vary. From the relation between the residual vibration amplitude and the frequency spectrum of feedforward control force, it is found that the control force can be robust for the natural frequency variations if the control force has no frequency component within the range of varying natural frequencies. In the proposed method, an enlarged system is first constructed in the modal coordinates for a virtual mechanism which has a few natural frequencies within the varying range of the concerned natural frequency. The optimal control theory is next applied to the enlarged system and the vibrationless feedforward control force is calculated. The improved robustness of the proposed vibrationless access control is demonstrated with experimental results.