An automatic document feeder includes a separating and feeding unit which has a separating member and a paper feeding member coming into contact with the separating member at a predetermined separating pressure, and separates and feeds an original sheet by sheet from a bundle of originals placed on an original placing table; a conveying unit which conveys the original separated by the separating and feeding unit toward a scanning position; a separating pressure switching unit which applies and releases the separating pressure and which is controlled by a control unit to release the separating pressure after a leading edge of a preceding sheet of the original has passed through the separating and feeding unit and to apply the separating pressure before a sheet of original separated and fed next to the preceding sheet of original is fed from the bundle of original to the separating and feeding unit.