The driving force can be transmitted at the time of both acceleration and deceleration in an engaged state where a first engagement face of a strut is forced to protrude inside a cutaway of a gear. The driving force is transmitted at the time of acceleration and the transmission of the driving force can be cut off at the time of deceleration in a one-way state where the first engagement face of the strut and the cutaway of the gear are made either to engage with or to disengage from each other in accordance with the rotational direction of the gear relative to the rotational shaft. The transmission of the driving force can be cut off at the time of both acceleration and deceleration in a disengaged state where the first engagement face of the strut is forced to withdraw from the cutaway of the gear. In addition, the decreasing of the transmission efficiency caused by friction between the strut and the gear can be prevented.