A plurality of engaging claws formed at a connecting member mounted to a rotary member of a driving unit via a one-way clutch are brought into engagement with a plurality of engageable grooves formed at a connecting member mounted to a rotated member of a driven unit so as to transmit the forward rotation of the rotary member to the rotated member. When the driven unit is mounted to the driving unit, the rotary member is forwardly rotated to bring at least one of the engaging claws into engagement with the engageable groove. Subsequently, the rotary member is reversely rotated to bring the respective engaging claws into engagement with the respective engageable grooves.