A sheet finisher of the invention includes a fold reinforcing unit that reinforces the fold line. The fold reinforcing unit includes a roller unit, a drive unit that has a single motor and moves the roller unit along the direction of the fold line from a standby position; and a nip unit. The nip unit includes a first nip plate and a second nip plate. The first nip plate and the second nip plate are opened in a thickness direction of the sheet bundle when the roller unit is located at the standby position, and are closed to nip the sheet bundle when the roller unit reinforces the fold line after the sheet bundle is transported to between the first and the second nip plate. The second nip plate is driven to be opened and closed in the thickness direction of the sheet bundle by the single motor.