This paper presents a force servoing control method to suppress torsional vibration of two-mass resonant system. The resonance ratio control is one of the effective control methods of two-mass resonant system. In this method, the ratio between the resonant frequency of motor and arm is determined arbitrary according to the feedback of estimated reaction torque. The reaction torque is estimated by using position sensitive detector (PSD). Since the estimation method does not need the parameter identification, the torsion information is obtained with accuracy. To attain the affinity and adaptability to environment, motion systems should control the reaction torque from the environment. In the force servoing system, the force command is given as a disturbance of the arm portion. The arm disturbance is observed by the arm disturbance observer. The proposed force servoing system is based on both the conventional PD control and the resonance ratio control and the determination method of pole placement is discussed. The proposed force servoing system can realize both the suppression of the inner torsional reaction torque and the adaptation to unknown outer force inputs. The numerical and experimental results show viability of the proposed method.