A rotation detection mechanism has a pulse plate, an optical sensor, a holder, and an input gear. The pulse plate has a light-blocking portion formed on an outer peripheral surface thereof. The optical sensor has a detection portion that detects whether an optical path thereof is opened or interrupted by the light-blocking portion of the pulse plate. The holder has a bearing hole that rotatably supports a rotary shaft of the pulse plate. The rotary shaft has a convex portion formed on an outer peripheral surface thereof and thus is in a key shape when seen from an axial direction thereof. On an inner peripheral surface of the bearing hole, a groove portion is formed through which the convex portion passes when the rotary shaft is inserted into the bearing hole, and the bearing hole is in a circular shape when seen from an axial direction thereof.