A sheet feeder includes: a wheel for issuing a lowermost sheet out to a sheet processing unit; a grate for spacing the sheet apart from the wheel at a raised position and bringing the sheet into contact with the wheel at a lowered position; and a drive device for driving the grate up and down. The drive device includes: a link mechanism for movably supporting the grate; a spring for urging the grate to the lowered position; a grate raising/lowering cam for periodically driving the grate up/down; and a skip feed cam that is driven by a control device, separately from a drive system of the grate raising/lowering cam, and that, at a timing where reached is a phase at which the grate raising/lowering cam does not put the grate in the raised position, retains the grate at the raised position in order to skip feeding of the sheet.