A sheet aligning device includes first and second aligning plates disposed on opposite lateral sides of the feeder board, the first and second aligning plates being capable of being reciprocatingly moved in a direction orthogonal to a sheet conveying direction and each having a sheet abutting surface parallel to the sheet conveying direction, an input member for inputting a sheet width, and a controller for controlling the first and second aligning plates so that the first aligning plate is moved, based on the inputted sheet width, to allow the sheet abutting surface thereof to be brought into abutment with a first lateral side of each sheet, thereby laterally aligning the sheet, and the second aligning plate is moved, based on the inputted sheet width, to allow a second lateral side of each sheet close to the second aligning plate to pass through a sheet-lateral-side passing zone on the second aligning plate. The sheet-lateral-side passing zone is located closer to the axis or centerline of the sheet than the sheet abutting surface is and has a flat surface throughout the sheet conveying direction.