A transmission device includes a driving motor, a separation roller module, a correction roller module, a discharging roller module, a first clutch, a second clutch, a third clutch and a plurality of sensors which are mounted to an automatic sheet feeder. The first clutch is disposed between the driving motor and the separation roller module. The second clutch is disposed between the driving motor and the correction roller module. The third clutch is disposed between the driving motor and the discharging roller module. The plurality of the sensors sense signal variations at the time of a leading end or a tail end of each of the documents passing through the plurality of the sensors, and the plurality of the sensors transmit the signal variations to the first clutch, the second clutch and the third clutch.