A sheet feeder comprises a sheet stacking unit which stacks sheets, a sheet pickup unit which picks up a sheet from one side of the stacked sheets on the sheet stacking unit, a sheet separating and feeding unit which separately feeds sheets one by one to a conveyance path, an orientation holding unit which holds a feed orientation of a sheet, and a biasing unit which biases the orientation holding unit from a retracted position to a projecting position. A projection amount of the orientation holding unit from an inclined surface increases as the number of stacked sheets decreases, and the projection amount from the inclined surface decreases as the number of stacked sheets increases.