PURPOSE: To respond to a sudden tension variation by providing a viscous elastic member on one side of the position where a fixing and a rotary members are opposed, and spherical members held and maintained in the condition squeezed rollable to the viscous elastic member between the positions the both members are opposed.

CONSTITUTION: To a sudden tension variation of a belt, the relative movement of a rubber member 21 and iron balls 22 is carried out while the iron balls 22 rotate in the condition squeezed to the rubber member 21, between the opening edge of the rear cap 1a of a fixing member 1 and the opening edge of the front cap 2a of a rotary member 2, and a viscous resistance by the rubber member 21 is generated in this case, so as to damp the rotation of the rotary member 2. As a result, adding to the damping function by the friction, a new damping force by the viscous resistance can be given, so as to obtain a large damping force. And since the viscous resistance is depending on the relative moving speed between the rubber member 21 and the iron balls 22, the damping force can be varied to a high damping side in the dynamic condition.