A folding device of the present invention includes a fold plate and a fold roller
pair for folding a sheet or a sheet stack conveyed thereto. A controller causes
the fold roller pair to move back and forth while nipping the folded portion of
the sheet or that of the sheet stack at its nip for thereby continuously exerting
pressure on the folded portion. The fold roller pair is rotated in opposite directions
for thereby sharpening the fold of the sheet or that of the sheet stack.
