A transmission device for driving an automatic document feeder (ADF) and a scanner individually is provided. The transmission device includes a transmission shaft, an actuator, a first transmission module coupled to the scanner, a second transmission module coupled to the ADF, and a power switching module. The actuator is movably attached around the transmission shaft. The power switching module is assembled to the transmission shaft. When the power switching module is coupled to the first transmission module and the transmission shaft rotates in a first direction such that the scanner moves to a first position, the scanner drives the actuator to drive the power switching module to be coupled to the second transmission module. When the transmission shaft rotates in a second direction, the power transmission module is coupled to the first transmission module to drive the scanner to move away from the transmission device.