SHEET FOLDING DEVICE, AND POST-PROCESSING DEVICE AND IMAGE FORMING SYSTEM INCLUDING THE SHEET FOLDING DEVICE

PROBLEM TO BE SOLVED: To provide a sheet folding device that prevents a sheet from being creased or abrasion damage from being brought about caused by folding rolls when folding the sheet with a pair of folding rolls, and which is inexpensive and has a simple structure.SOLUTION: The sheet folding device includes: a sheet bundle supporting means for holding a sheet bundle at a predetermined folding position; a pair of folding rolls arranged at the folding position; a folding blade for inserting the sheet bundle in a nip position between the pair of folding rolls; a carrying-out roller arranged downstream from the pair of folding rolls and carrying out the folded sheet bundle from the folding position; a roll driving means for rotating the pair of folding rolls in a sheet folding direction; a blade driving means for reciprocating the folding blade between the nip position and a standby position where the folding blade retreats from the nip position; and a drive control means for controlling the roll driving means, the blade driving means and the carrying-out roller.