A retard roller of an automatic document feeder provides a frictional force to separate plural documents from each other. The retard roller can provide a first damping torque and a second damping torque with different directions. The second damping torque is smaller than the first damping torque. As such, the frictional force exerted on the document is reduced for returning the document out of the automatic document feeder. A retard roller module having such a retard roller is also provided.