In recent years, although there are many researches on bilateral control, there is little research on improvement of operationality. It will be so effective for operators to improve the operationality of the system in bilateral control. In this paper, a new improving method of operationality which is based on design of nominal mass in disturbance observer is proposed. First, disturbance observer which is implemented in the system is introduced. Then, the relationship between observer gain and nominal mass variation in disturbance observer is discussed. Secondly, bilateral control for transmitting of vivid force sensation is proposed. Then, the proposed method is applied to bilateral control. Finally, the experimental results show the viability of the proposed method.