Recently, interactions between humans and robots have been changed in order to solve the problems of the aged society. Opportunities that robots touch directly on humans have been increasing. However, active actuators are often used in order to generate force and perform their tasks and they have possibility of hurting people around. Then, stable passive actuators would be substituted for active ones. Passive actuators will not generate force by themselves. Only when external force acts on them, they generate resisitive force. Considering these properties, passive actuators are stable actuators and appropriate for the human society because they would be little risks for humans when they are loss of control. In this paper, the passive actuator is constructed using magnethorheological (MR) fluids and is nominalized based on disturbance observer in order to become robust for inner state of MR fluid and to be regarded as an active actuator. By experiments, the validity of the proposed method is verified.