A bilateral control robot is one of the master-slave teleoperation robots. Consider two robots, a master robot is manipulated by human operators and a slave robot contacts remote environment. Decoupling of a force control and a position control in bilateral control is realized using the mode transformation i.e. a force controller in the common mode and a position controller in the differential mode. In conventional method, high transparency in a bilateral control is obtained when the inertia of the master and slave robots are equal. However, high transparency is not achieved when the inertia of the master and slave robots is different. Additionally, transparency goes down in the case of the bilateral control with scaling. Since the common mode and the differential mode interfere when the inertia and scaling of the master robot slave robots are different. In this paper, modal space disturbance observer (MSDOB) is proposed to solve these interferential problems. MSDOB is disturbance observer in the modal space and is implemented in each mode. MSDOB eliminates these interferences and modeling error in the modal space. In the results, decoupling of the force control and the position control is realized. Therefore, high transparency is achieved even if the inertia and the scaling of the master and slave robots are different. From the simulation and experimental results, the validity of the proposed method was confirmed.