A sheet feeder feeds a sheet to an image forming unit. The sheet feeder can include a supply tray that holds a stack of sheets, a separation element disposed at an end of the supply tray, and configured to contact and apply a resistive force to a sheet in the stack, and a separation roller disposed facing the separation element, and configured to rotate, contact and apply a feeding force to an uppermost sheet in the stack. The sheet feeder may also include a pressing plate pivotally attached to the supply tray at an end remote from the separation element, and configured to move toward the separation element as a number of sheets in the stack decreases, and a pressure applying element that increases a contact surface pressure between the separation element and the uppermost sheet in response to the pressing plate moving toward the separation element.