Tendon-driven rotary actuators are utilized in various robots because they are small and high output. However, if the tendon-driven rotary actuators are utilized in bilateral control system, there are two problems. The first problem is interference. Bilateral control systems with tendon-driven mechanisms require decoupling of angle tracking, torque feedback, and tension control on master side, and slave side. The second problem is elongation. The tendons are extended by applied force. Therefore, it is difficult to control the angle precisely. In this paper, a method to compensate the error caused by the tendon elongation for tendon-driven mechanisms is proposed. This compensation method is designed based on modal transformation. The validity of the proposed method is verified by an experiment. Elongation is reduced by the proposed method. In addition, angle tracking, torque feedback, and tension control on master side, and slave side are achieved independently. Experiments were conducted to confirm the validity of the proposed method. In this experiment, proposed method is applied to a bilateral control system.