The technology in robotics has been developed remarkably in recent years. Especially, usage of robots in medical field, such as telesurgery, is expected more and more along with the population aging. To realize precise surgery, accurate transmission of force sensation is an important issue. Therefore, bilateral control system is taking attention for the application of robots to telesurgery. Bilateral control system consists of motors and encoders. To reduce the weight of robots, the flexible actuator, which is composed of direct drive motor and thrust wire, is a useful device. However, in multi-DOF robots, disturbance force occurs to move the thrust wire and the operability of the robot deteriorates as a result. This paper proposes a spring and damper model to estimate such disturbance force. Estimated disturbance force is evaluated comparing with experimentally obtained values.