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PID controller, disturbance observer, and so on have been applied to control of servo systems. However, it is difficult to know which controller is the best for given control object and specifications. If we want to realize a best performance among many controllers, we have to design each controller independently. In this paper, we derive a combined controller that is based on the disturbance observer structure. Parameters of the combined controller are designed by some specification constraints for robust stability, robust performance, command response, and sensitivity to disturbance and noise. This method is applied to a rigid body system and a two-mass system under some different specifications. Different controller structure is selected for each specification. From the result, we show a performance comparison table of conventional controller structures for each specification., which is useful to select a suitable structure controller for given control object and specifications.