A transmission device for an automatic document feeder includes a driving gear set driven by a driving motor, a first swing arm equipped with a first driving roller set, a second swing arm equipped with a second driving roller set, and a one-way gear set driven by the driving gear set to keep transmitting rollers rotating in the same direction. The first swing arm is driven by the driving gear set and swingable between a first position and a second position to mesh the first driving roller set with a pickup device or an output device. The second swing arm is driven by the driving gear set and swingable between a third position and a fourth position to mesh the second driving roller set with the pickup device or the output device. When the first swing arm swings toward the first/second position, the second arm swings toward the fourth/third position.