A control method for improving the tracking performance with respect to the reference trajectory has been developed. The proposed control system has an inner/outer control structure. The inner control system is the existing control system based on the conventional PID controller, which is designed in advance, independent of the outer control system. In order to improve the tracking performance, the outer control system is designed based on optimal preview control theory and is added to the outside of the inner control system. The polynomial trajectory is used as the reference trajectory and is fed to the outer control system. The reference trajectory is modified on-line to minimize the tracking error. The modified reference trajectory is derived as a control input signal of the outer control system and is fed to the inner control system. The effectiveness of the proposed control method is confirmed by numerical simulations of the benchmark problem for hard disk drives (HDDs).