There are two evaluation indices for bilateral control systems based on transparency, which are "Reproducibility" and " Operationality". Reproducibility is degree of reproduction of the environmental impedance in the master side. Operationality is degree of operational force which a human operator feels besides reaction force from environments. It is desirable that these evaluation indices are improved as much as possible. In this research, a master-slave manipulator which is controlled by a bilateral controller with disturbance observers is described. This paper shows that reproducibility and operationality of the bilateral system are improved based on design of nominal mass which is a parameter of a disturbance observer. It is possible to improve reproducibility and operationality simultaneously with an appropriate nominal mass design. Furthermore, variation of stability of the system because of variation of nominal mass is analyzed with root locus. Validity of this research is confirmed by experimental results.