In some industrial motor drive systems, a torsional vibration is often generated because of an elastic element in torque transmission. Such mechanical system is modeled as a two-mass-system and it is well-known that the suppressing vibration of a low inertia ratio two-mass system where the motor inertia is larger than the load inertia is very difficult. This paper proposes a speed control system of a low inertia ratio system taking not only the dynamic responses but also a robust stability into account. The proposed control system based on the H^∞ control theory and the resonance ratio control due to the feedback of the estimated shaft torque. Combining the H^∞ controller with the resonance ratio controller, the control system with high robust stability can be obtained comparing with the conventional resonance ratio control. The variable feedback gain system and the construction of the disturbance observer are discussed in order to reject the effects of noise. The simulated and experimental results show that the proposed speed control system is useful for the two-mass-system with low inertia ratio.