Sheet feeder and image forming apparatus

A sheet feeder, comprising:

• an attraction separation unit including an attraction belt stretched between rollers and disposed to face the upper surface of a stacked sheet bundle to attract an uppermost sheet of the sheet bundle, rotatably supported by a support member at a position upstream of the attraction belt in a sheet feeding direction wherein a downstream one of the rollers is movable relative to the support member;
• a swing device to swing the attraction separation unit around the support member as a pivot to move the attraction belt back and forth between an attraction position for attracting the uppermost sheet of the sheet bundle to the attraction belt and a feed position for feeding the uppermost sheet attracted to the attraction belt that is more distant from the sheet bundle than is the attraction position, and configured to swing the attraction belt to the feed position to be rotated to feed the uppermost sheet after the uppermost sheet has been attracted to the attraction belt at the attraction position;
• and
• a biasing device to bias the downstream one of the rollers in the sheet feeding direction toward the sheet bundle.