A sheet processing apparatus includes a sheet conveying unit configured to convey sheets, a first scanning unit configured to scan a surface of each of the sheets conveyed by the sheet conveying unit, and generate a first image data for each of the sheets, an erasing unit configured to carry out an erasing process on the surface of each of the sheets conveyed from the first scanning unit, a second scanning unit configured to scan the surface of each of the sheets conveyed from the erasing unit, and generate a second image data for each of the sheets, and a control unit configured to cause the sheets to be sorted based on a difference between the first image data and the second image data thereof.