A coupling unit includes an output gear, a coupling and a rubber cushion. A second rotatable shaft portion of the coupling includes a resilient projection that projects outwardly in a radial direction from an outer peripheral surface of the second rotatable shaft portion. The resilient projection is resiliently inwardly deformed in the radial direction during movement of the resilient projection along a deep side of a recessed groove of a through-hole formed in a first rotatable shaft portion of the output gear at a time of fitting the second rotatable shaft portion into the through-hole. When the resilient projection projects from the through-hole, the resilient projection is resiliently outwardly restored and is engaged to an engaging portion of the first rotatable shaft portion to limit removal of the second rotatable shaft portion from the through-hole.