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The disturbance observer (DOB) is known to be effective in enhancing the performance of dynamic systems in the presence of disturbances. In general, the disturbance rejection performance and robustness of DOBs depend on the order of the Q-filter, which is a low pass filter, and its time constant. In this paper, a new multi-loop DOB, improving the performance of DOB, is presented and its robust stability is proven using linear fractional transformation. It is possible to improve the sensitivity function with additional external-loop DOBs which do not significantly affect the complementary sensitivity function. The proposed new multi-loop DOB is applied to a commercial optical disk drive tracking servo system. Experimental results show that the proposed multi-loop DOB is an effective method in rejecting disturbance as well as improving the tracking performance.