Outer plates have a doughnut-like disk shape and are attached to the inner circumferential surface of a case member. On the other hand, inner plates have a substantially arc shape and are swingably supported about a position at a distance from the center of rotation of a shaft. The inner plates are held at a first position, at which the interlocking area between the outer plates and the inner plates is small, by a tension coil spring when the relative rotational speed between the case member and the shaft is small, and, when the relative rotational speed between the case member and the shaft exceeds a predetermined value are pivotally moved toward the outer plates by the shearing force of a viscous fluid and held at a second position at which the interlocking area between the movable plates and the first plates is large.