Safety operatons are one of most important issues in human society, when robots are introduced there. Recently, human-assist systems acting directly on humans have been developed and opportunities for humans to touch robots are increasing. However, active actuators are often used to generate force in order to perform any tasks and they constitute a possible danger to humans when they are loss of control. Then, stable passive systems would be substituted for active ones. Passive systems will not generate force by themselves. Only when external force acts on them, they generate reaction force. Considering these properties, passive systems are stable 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 and the active actuator are combined together in order to generate the torque. The passive system is used like a clutch. By control of the current of passive system, the output torque from MR-fluid actuator is also controlled. The maximum output of MR-fluid actuator is limited by shear stress of MR fluids. By these properties, MR-fluid actuator has high safety for humans. In this paper, the velocity control of MR-fluid actuator is proposed.