An automatic sheet feeder comprises a feed roller which feeds sheets stacked in a pile one by one to a sheet conveying path, a conveyor roller provided on the sheet conveying path to convey the sheet delivered to the sheet conveying path by the feed roller to an image reading unit. The sheet conveying speed of the conveyor roller is higher than the sheet feeding speed of the feed roller. A feed roller gear is mounted on a feed roller shaft fixedly supporting the feed roller thereon and is interlocked with the feed roller shaft by a one-way clutch mechanism. The clutch mechanism couples the feed roller gear with the feed roller shaft to drive the feed roller for rotation by the feed roller gear until the leading edge of the sheet arrives at the nipping line of the conveyor roller. Thereafter the feed roller shaft is uncoupled from the feed roller gear so that the feed roller is rotated through the sheet by the conveyor roller after the leading edge has arrived at the nipping line of the conveyor roller. The feed roller gear and the feed roller shaft are again coupled in a fixed time after the trailing edge of the sheet has left the feed roller.