A sheet processing apparatus which flattens a curled sheet includes: a water content control unit which controls a water content of a sheet to flatten the curled sheet; a decurling section which is provided downstream the water content control unit with respect to a conveyance direction of the sheet, and the decurling section bends the curled sheet to be flat by a bending force; and a conveyance route changing section which changes a conveyance route of the sheet to direct to the water content control unit or not.
