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 supporting means which supports a sheet bundle at a predetermined folding position; a pair of folding rolls brought into press-contact with each other, which is arranged at the folding position, and folds the sheet bundle; a folding blade which inserts the sheet bundle in a nip between the pair of folding rolls; a roll driving means which rotates the folding rolls in a sheet folding direction; a blade driving means which reciprocates the folding blade between a standby position and the nip position between the pair of folding rolls; a pair of paper ejection rollers which is arranged downstream from the pair of folding rolls, and carries out the folded sheet bundle; and a drive control means which controls the roll driving means, the blade driving means and the pair of paper ejection rollers.