Abstract: A novel rotary actuator utilizes viscosity of the oil and prevents the oil leakage, without using the sealing member to the vane. The instant device installs the flanges in parallel and fixes the vane therebetween, and sets within the cylinder the rotor keeping the stopper therebetween, not fixing it, and the radial bearings at both sides of the cylinder to support the stopper, whereby a fine clearance is maintained between the flange and the inner wall of the cylinder. The rotor is formed in order of machining into flanges, small diameter portions and a rotary shaft. The vane is formed by cutting the material of doughnut shape into a sector.