An automatic document feeder includes a first roller portion on the first transportation path to feed an original to a scanning position to scan a first face of the original, a second roller portion on a second transportation path to transport the scanned original, a third roller portion on the second transportation path to feed the original to the scanning position to scan a second face of the original, a fourth roller portion on the first transportation path to transport the scanned original, and a transporting speed adjusting unit gaining a circumferential velocity of the second roller portion than the one of the first roller portion and gaining a circumferential velocity of the fourth roller portion than the one of third roller portion, when the original is transported from the first transportation path to the second transportation path and from the second transportation path to the first transportation path, respectively.