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This paper presents a design procedure of H-infinity robust controller of three dimensional two links flexible robot arm taking account of posture change and uncertain payload. Bending and torsional coupled vibration characteristic of the arm is fluctuated by posture change and uncertain payload. Two-degree-of freedom (2DOF) model is applied to denote the two links flexible robot arm. Parameters of the model is identified experimentally by utilizing Seto's modeling method. To design robust H-infinity controller, two models are identified. Nominal model case denotes the state that two links are extended straight, while fluctuated model case denotes the state that two links are vertical to each links. Robust servo controller design procedure is introduced that applies the H-infinity control theory in consideration of the parameter change due to the posture change and holding payload. The H-infinity controller is obtained using a generalized plant including error matrices and two filters; Error matrices represent the fluctuation of parameters between nominal and fluctuated model. One filter is high-pass filter to avoid spilover, and the another filter is low-pass filter with integral characteristic to be used as an pseudo-integrator to achieve servo control. The effectiveness of the obtained controller are verified by the simulation and the control experiment.