This paper proposes a disturbance observer (DOB) design method for bilateral control systems with multi-degree-of-freedom (MDOF). DOB is one of the robust control methods based on acceleration. DOB uses the nominal mass or nominal inertia for the estimation of disturbance force. A control performance of the bilateral control with DOB is changed according to the design of the nominal mass in DOB. This paper firstly discusses the relationship between nominal mass in DOB and the control performance. Then, the design method of bilateral control with low nominal parameter is proposed. The effectiveness of proposed method is confirmed from the experimental results.