In bilateral control system under time delay, contact motion becomes unstable. Damping injection is the method to stabilize contact motion. However, large operational force is induced by damping injection and operationality is deteriorated. To solve this deteriorating, this paper proposes damping injection without deteriorating operationality. In proposed method, free motion and contact motion are identified by using position-based contact detection. Therefore, damping injection can work only at the time of contact motion and large operational force does not occur. The effectiveness of the proposed method is confirmed by some experiments.