A cutter unit (10) is provided, composed of a stationary cutter (11) installed between staple legs and a movable cutter (13) operated from the outside of the staple legs toward the inside. The cutter unit (10) is installed so as to be slidable between a position at which the unit is advanced in an operation region of a movable clincher (5) and a position at which the unit is receded from the operation region of the movable clincher (5). The cutter unit (10) is advanced into a staple hammer-out section to cut the staple legs penetrated through paper sheets to be bound, the cutter unit (10) is receded from the operation region of the movable clincher (5), and then the movable clincher (5) is caused to pivot to bend the staple legs.