A staple leg cutting mechanism 5 is composed of a fixed cutter 6 interposed between staple legs S1 penetrated through binding sheets and a pair of movable cutters 7 disposed movable from the outside of the staple legs S1 to the inside thereof, and a clincher mechanism 10 is composed of a pair of movable clinchers 11 respectively disposed on the lower surface side of their associated movable cutters 7, whereby the staple legs S1 penetrated through the binding sheets are formed in an inwardly curved manner by the movable clinchers 11 and are then guided into and between the movable cutters 7 and fixed cutter 6, the movable cutters 7 are moved from the outside of the staple legs S1 to the inside thereof to thereby cut the leading end portions of the staple legs S1, and the staple legs S1 are bent along the binding sheets by the movable clinchers 11.