A stapler includes a staple cartridge in which a staple is stored, a storage unit to which the staple cartridge is detachably attached, a staple ejecting unit which ejects the staple to penetrate a paper sheet, a cutting unit which cuts a staple leg of the staple penetrating the paper sheet, a binding unit which bends the staple leg of the staple penetrating the paper sheet to bind the paper sheet, a cut staple storage unit which stores a cut staple that is cut by the cutting unit, a cut staple quantity detection unit which detects a quantity of the cut staple stored in the cut staple storage unit, and a discharge unit through which the cutting unit and the cut staple storage unit communicate with each other to guide the cut staple to the cut staple storage unit. The cut staple storage unit is attachable to and detachable from the stapler.