An automatic document feeder includes a feeding path and a plurality of feed roller sets. Through the feeding path, an original document is fed in one direction from a paper supply port to a paper discharge port. The plurality of the feed roller sets are arranged along the feeding path to be spaced apart from each other. When one side of an original document is scanned and when both sides of an original document are scanned, the plurality of the feed roller sets nip the original document to feed the original document along the feeding path. In this case, the plurality of the feed roller sets are all involved in feeding the original document.