A sheet folding device has a releasable clutch between a roll driving device and a pair of folding rolls for folding a sheet bunch so that when a folding blade inserts the sheet bunch to a nip position on the pair of folding rolls, the first and second folding rolls rotate following the inserted sheets. The sheet folding device includes a guide for holding the sheet bunch at a predetermined fold position. The first and second folding rolls arranged at the fold position are in pressure contact with each other. The folding blade inserts the sheet bunch supported on the guide to the nip position on the first and second folding rolls. The roll driving device rotationally drives the first and second folding rolls. The first and second folding rolls and the roll driving device are coupled together via the releasable clutch.