Recent machines are hoped to have a haptic ability to work in open environment. Haptic motion control is based on position control and force control. To realize the position control and especially force control, the attainment of robustness is essential. This paper selects a disturbance observer to attain the robust acceleration control. A disturbance observer identifies the total mechanical load torque and parameter variations. The aim of this paper is to analyze a disturbance observer with the focus on parameter variations. Parameter variations have an influence on the cut-off frequency of bandwidth where robustness is assured and the stability of control system. This paper shows that the cut-off frequency depends on parameter variations. This paper focuses on the self-inertia variation and analyzes the stability of the position control system and the force control system. The design method of observer gains is also proposed.