A document feeder includes a document feeding means which feeds document sheets stacked on a document tray one at a time, a registration means which feeds each document sheet to an image reading position, a re-feeding means which receives the document sheet after a first side thereof is read and re-feeds the document sheet to allow a second side to be read, a discharging means which discharges the document sheet, a first drive transmission mechanism which transmits a driving force to the document feeding means and the re-feeding means, and a second drive transmission mechanism which transmits the driving force to the registration means and the discharging means. The driving force is transmitted to the first and second drive transmission mechanisms from a single motor. A clutch is provided upstream of the document feeding means in a direction in which the driving force is transmitted in the first drive transmission mechanism.