To perform proper and stable paper feed requesting a device manager to have neither an advanced technology nor excessive labor, with regard to a sheet feeder.

The sheet feeder 3 including a paper feeding tray 20 that loads a paper bundle, a paper feeding roller 23 where the paper is sent from the paper bundle in the paper feeding tray 20 by being driven for rotation by a motor 15, a separating plate 22 whose upper face is oppositely disposed to be able to keep close contact with a peripheral surface of the paper feeding roller 23 together with the paper feeding roller 23 constitutes a separating mechanism 21, and a sending roller 25 that conveys sheets of the paper that are rotated and driven by a drive means and further conveyed downstream of the separating mechanism 21 to be sent further downstream, and controlled for drive by a sub-controller 11 and the sheets of the paper are supplied one by one where a pressure sensor 14 that detects pressure generated between the paper feeding roller 23 and the separating plate 22 as separating pressure and outputs the detected signal to the sub-controller 11 is installed in the separating mechanism 21 and controls the separating pressure on the basis of separating pressure data detected by the sub-controller 11 is provided.