A sheet feeder of a corrugated paperboard box making machine includes a sheet feed table, feed rolls, an adjusting mechanism, a main unit, and a backing roller. The adjusting mechanism holds support plates pivotally supporting both ends of a first feed roll opposed to a second feed roll having an axis fixed to the main unit so as to allow an axis of the first feed roll to move with respect to the main unit. The support plates support a first backing roller via a connecting member connecting the support plates so that the first backing roller contacts with an outer peripheral surface of the first feed roll.