The invention relates to an automatic document feeder, an image reading device, an image forming apparatus, and an image forming system. The automatic document feeder comprises: a first conveyance portion through which documents from a bundle of documents placed on a paper feeding tray are separated from one another and conveyed to a reading position; a second conveyance portion through which a document having been conveyed to the reading position is ejected onto a paper ejection tray; a detector that detects any one mode from a productivity mode and a silent mode; and a controller that controls, when the detector has detected the productivity mode, the first conveyance portion so as to bring a conveyance speed in the first conveyance portion to a first conveyance speed, and, when the detector has detected the silent mode, controls the first conveyance portion so as to bring a conveyance speed in the first conveyance portion to a second conveyance speed that is slower than the first conveyance speed.