A sheet feeder, comprising: a plurality of wheels which are repeatedly operated and stopped and feed a lowermost sheet among stacked sheets to a sheet processing section during an operation; a grate which separates the lowermost sheet from the respective wheels at a raised position and causes the lowermost sheet to come into contact with the respective wheels at a lower position; and a drive device which raises and lowers the grate in cooperation with operation of the respective wheels, wherein the drive device includes a link mechanism which movably supports the grate, a spring which engages with any link of the link mechanism and urges the grate to the lowered position, a grate raising/lowering cam which has a cam surface which can abut on any link of the link mechanism and periodically raises and lowers the grate to the raised position when the respective wheels stop and to the lowered position when the respective wheels are operated, the sheet feeder being characterized by a skip feed cam which has a cam surface which can abut on any link of the link mechanism, is driven by an electric motor separately from a drive system of the grate raising/lowering cam, is operated at the time of a skip feed mode, and maintains the grate at the raised position at the timing becoming a phase where the grate raising/lowering cam does not position the grate at the raised position so as to skip-feeds the lowermost sheet, and control means for stopping the skip feed cam in a state where the cam surface is separated from a link of the link mechanism at the time of a normal mode and operating the skip feed cam at the time of the skip feed mode. a plurality of wheels which are repeatedly operated and stopped and feed a lowermost sheet among stacked sheets to a sheet processing section during an operation; a grate which separates the lowermost sheet from the respective wheels at a raised position and causes the lowermost sheet to come into contact with the respective wheels at a lower position; and a drive device which raises and lowers the grate in cooperation with operation of the respective wheels, wherein the drive device includes a link mechanism which movably supports the grate, a spring which engages with any link of the link mechanism and urges the grate to the lowered position, a grate raising/lowering cam which has a cam surface which can abut on any link of the link mechanism and periodically raises and lowers the grate to the raised position when the respective wheels stop and to the lowered position when the respective wheels are operated, the sheet feeder being characterized by a skip feed cam which has a cam surface which can abut on any link of the link mechanism, is driven by an electric motor separately from a drive system of the grate raising/lowering cam, is operated at the time of a skip feed mode, and maintains the grate at the raised position at the timing becoming a phase where the grate raising/lowering cam does not position the grate at the raised position so as to skip-feeds the lowermost sheet, and control means for stopping the skip feed cam in a state where the cam surface is separated from a link of the link mechanism at the time of a normal mode and operating the skip feed cam at the time of the skip feed mode.