Electro-rheological materials, such as electro-rheological fluid and electro-rheological elastomer, are functional materials which change these viscoelastic properties according to the electrical field intensity. It is expected to apply to the machine element, such as brake and clutch. Meanwhile, sensor-less control system of application devices using ER material is required to cut cost in practical use. In this study, sensor-less viscous torque control of ER passive actuator using parallel disturbance observer (DOB) is proposed. Parallel DOB can estimate the viscous torque from both applied voltage and angular velocity, and realize robust torque control without additional torque sensor. The effectiveness of this method is confirmed from simulation and experiments.