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This paper describes the construction of an observer-based brushless DC motor drive control system with a mechanical resonant load and an optimum design method. At first, the construction of a motor drive control system which has a load speed feedback loop using a new type of observer is proposed. The observer has two features. The first is that it is possible to detect the load speed accurately even if the load disturbance torque changes. The second is that it is not necessary to allocate the roots of the observer. For this reason, it is very easy to apply this observer to the servo system with a mechanical resonant load as a component of a load speed feedback loop. Next, its design method, to which is applied a parameter ultimate sensitivity method based on the partial model matching method, is proposed. This method has a feature that it is possible to secure both the response and the stability of the servo system by setting only one parameter, α. All roots of the characteristic equation of the servo system can be allocated inside the fan shaped region located on the negative side of the real axis in the complex plane under all conditions. Finally, from the simulation results, the effectiveness of the construction of the observer-based servo control system and its optimum design method are proved adequate.