A sheet folding device for folding a sheet or a sheet stack conveyed thereto of the present invention includes a path along which the sheet or the sheet stack to be folded is conveyed. A fold plate is movable in a direction perpendicular to the above path for forming a fold in the sheet or the sheet stack. A pair of rotatable fold rollers face the fold plate and are positioned one above the other. Pressing members exerts, when the fold is to be formed, a pair of equal pressing forces on the fold rollers to thereby maintain the nip between the fold rollers on a line including the locus of movement of the fold plate.