There is provided an image reading device including a power transmission unit that transmits power of a driving source to a transportation roller and a discharging roller, in which the power transmission unit is configured to include a first gear group that constitutes a first power transmission route and a second gear group that constitutes a second power transmission route and that is configured to include a smaller number of gears than the first gear group, the second gear group includes a transportation roller driving gear and is disposed inside a curved inversion route as seen from an axial direction of a rotation shaft, and at least a portion of the transportation roller driving gear is positioned inside a medium transportation region in a width direction of a medium.