A stapler including: cut,ting-off means for cutting off a staple positioned at a leading portion of interlinked staples from the interlinked staples in which a plurality of approximately straight lined paper-made staples are interlinked in parallel; shaping means for shaping the staple cut off by the cutting-off means such that a crown portion and leg portions bent approximately perpendicularly from the right and left sides of the crown portion are formed; penetration means (8, 18) for penetrating both the leg portions of the staple shaped by the shaping means through binding sheets; and bending means (23, 24, 28, 29) for bending both the leg portions of the staple penetrated through the binding sheets by the penetration means along the binding sheets and for bonding them mutually, characterized in that the penetration means includes: a pushing unit for pushing down the staple with respect to the binding sheets; and two pieces of insertion-cutting blades (51a, 51b) which are provided concurrently at an interval in response to a length of the crown portion and which are provided with protrusion portions (51ad, 51bd) formed by protruding portions in the width direction at a predetermined height of the surfaces facing each other, and the respective insertion-cutting blades penetrate the binding sheets in a state in which both the leg portions of the staple that is shaped by the shaping means into a shape in which both the edges thereof are bent to one direction are attached on the surfaces facing each other with respect to the respective insertion-cutting blades on the upside of the respective protrusion portions and the staple is pushed down by the pushing unit with respect to the binding sheets. cut,ting-off means for cutting off a staple positioned at a leading portion of interlinked staples from the interlinked staples in which a plurality of approximately straight lined paper-made staples are interlinked in parallel; shaping means for shaping the staple cut off by the cutting-off means such that a crown portion and leg portions bent approximately perpendicularly from the right and left sides of the crown portion are formed; penetration means (8, 18) for penetrating both the leg portions of the staple shaped by the shaping means through binding sheets; and bending means (23, 24, 28, 29) for bending both the leg portions of the staple penetrated through the binding sheets by the penetration means along the binding sheets and for bonding them mutually, characterized in that the penetration means includes: a pushing unit for pushing down the staple with respect to the binding sheets; and two pieces of insertion-cutting blades (51a, 51b) which are provided concurrently at an interval in response to a length of the crown portion and which are provided with protrusion portions (51ad, 51bd) formed by protruding portions in the width direction at a predetermined height of the surfaces facing each other, and the respective insertion-cutting blades penetrate the binding sheets in a state in which both the leg portions of the staple that is shaped by the shaping means into a shape in which both the edges thereof are bent to one direction are attached on the surfaces facing each other with respect to the respective insertion-cutting blades on the upside of the respective protrusion portions and the staple is pushed down by the pushing unit with respect to the binding sheets. a pushing unit for pushing down the staple with respect to the binding sheets; and two pieces of insertion-cutting blades (51a, 51b) which are provided concurrently at an interval in response to a length of the crown portion and which are provided with protrusion portions (51ad, 51bd) formed by protruding portions in the width direction at a predetermined height of the surfaces facing each other, and the respective insertion-cutting blades penetrate the binding sheets in a state in which both the leg portions of the staple that is shaped by the shaping means into a shape in which both the edges thereof are bent to one direction are attached on the surfaces facing each other with respect to the respective insertion-cutting blades on the upside of the respective protrusion portions and the staple is pushed down by the pushing unit with respect to the binding sheets.