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SUMMARYA well-known control system that can reduce the adverse effects of disturbances is the disturbance observer. Whenever we apply a disturbance observer, the disturbance frequency should be known and constant. However, since in many cases of industrial systems the disturbance frequency is varied in some frequency spectrum, it may be difficult to reduce the adverse effect of such disturbances by use of the traditional disturbance observer. Therefore, we have proposed a disturbance observer which can handle frequency variation (DOFV). However, in order to apply DOFV we must measure the disturbance frequency in real time. Therefore, a frequency estimation scheme is required in order to apply DOFV. In this paper, a method for the design of a disturbance attenuation system that can handle frequency variation is proposed. The main idea of this design method is to combine DOFV with a frequency estimator that can estimate the disturbance frequency in real time. Even though the proposed disturbance attenuation system is a low-degree and low-gain controller, it has excellent steady-state disturbance attenuation performance. Numerical simulations and experiments show the usefulness of the proposed disturbance attenuation system.