Force display systems are a kind of human-coexistent robot systems, which share the space with people while working, and which directly touch and display force-senses to their users. They are expected to have much effects and advantages as means of human interface. On the other hand, they are inherently involved a potential hazard in that they may uncontrollably move and hurt operators. We have proposed that safety of robot systems can be estimated quantitatively and ensured mechanically by using a clutch-type actuator. Moreover, we have developed ER actuators and MR actuators that are a kind of clutch-type actuators. In this study, we successed to develop an MR actuator with low inertia, high responsibility, and high torque/inertia ratio, and to develop a 2-D force display system using the developed MR actuator that can present virtual objects with high rigidity and good operativeness.