PURPOSE: To eliminate generation of paper jamming by providing a groove or a flat part extending in the axial direction of a flap type biting body between adjacent flaps of the flap type biting bodies, and arranging the groove or the flat part so that a blade strip or a grooved beam is located opposite to the groove or the flat part in passing the body clearance.

CONSTITUTION: A groove 18 or a flat part is provided on an outer circumferential surface of a flap type biting body 12 in a region located opposite to a blade strip 8 or a grooved beam (a region between adjacent biting flaps 13). The body gap is increased when the groove 18 or the flat part passes through the body gap between a piercing and folding blade body 6 and the flap type biting body 12, and as a result, the stacked papers can pass the body gap without any risk.