Nowadays, robots manipluated by human are aggressively researched and developed. Such robots need large generative force, high speed response, good controllability, high safety, low friction, backdriveability and so on. We have put emphasis on the backdriveability and created a novel prototype linear actuator with new structure applying an ER (Electro-Rheological) gel, a recent functional material. It is an all-purpose actuator satisfying characteristics for human-coexistence welfare robot.