Along with the population aging, human interactive robots are expected to be introduced in our daily lives especially in developed countries. For such robots, bilateral control is widely researched as a technique for transmission of both position and force information. Reaction force/torque observer (RFOB/RTOB) is often utilized in bilateral control system to estimate reaction force/torque from objects. It achieves a sensorless control system. However, it requires accurate estimation of operational force/torque for precise approximation of reaction force/torque. This paper proposes a method for estimation of operational torque. In this method, a hysteresis model is utilized to reduce vibration which occurs when the velocity shakes at low velocity. The validity of the proposed method is confirmed by experimental results.