An automatic document feeder includes a document stacker for holding documents, a first conveyer for feeding the documents one sheet by one sheet to a platen glass for reading an image on a first side of the conveyed document, a second conveyor for conveying the conveyed document so that the conveyed document is turned over and conveyed to the platen glass again to read an image on the second side, and a discharger for discharging the conveyed document from the platen glass and returning the conveyed document to the document stacker. A third conveyer, located over the platen glass, conveys the conveyed document from at least one of the first conveyor and the second conveyer to the platen glass and conveys the conveyed document from the platen glass to at least one of the first conveyer, the second conveyer, and the discharger. A controller controls the movement of the conveyed document by the first, second, and third conveyers and the discharger, and a next conveyed document following the conveyed document is set to a waiting position on the platen glass when a second circulation is executed by the controller following a first circulation. The next conveyed document is then conveyed to the platen glass through the second conveyer after the second side of the conveyed document is read.