Bilateral control system is widely researched as a technique for robotic telesurgry, since reaction force from objects is important information for precise surgical operation. Reaction force/torque observer (RFOB/RTOB) realizes sensorless estimation of external force and it has been efficiently utilized for bilateral control. However, precise estimation of operational force is required for accurate estimation of reaction force. Otherwise the operability of the robot deteriorates. In addition, operational force, such as friction force, changes within the range of motion when the structure of the robot is asymmetric. This paper proposes a method for estimation of operational torque considering of the asymmetric property. The validity of the proposed method is confirmed by experimental results.