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