A sheet feeder has a plurality of sheets supported on an intermediate plate, and feeds sheets separately one by one by attracting them by a suction belt. Air is blown from a blowing duct through an opening toward a side edge of a sheet bundle supported by the intermediate plate in order to handle the sheet bundle, and so as to vary the speed and/or direction of air blown from the opening to reliably handle the sheet bundle, whereby a sheet conveying operation by the suction belt is performed when the speed of air blown on the side edge of the sheet located in the uppermost position bundle is not more than a predetermine value.