The method of forming a through-hole is capable of preventing formation of burrs, improving machining efficiency and reducing machining cost. The method of forming a through-hole in a circular wall of a cylindrical part of a work piece comprises the steps of: setting the work piece in a die; inserting a punch, which is provided to a rod-shaped metal core and whose length projected from the metal core is shorter than thickness of the circular wall, into the cylindrical part; relatively pressing and moving the punch toward the die so as to drive the punch into an inner face of the circular wall and bore the through-hole; sucking a scrap, which is formed by boring the through-hole, via a discharge hole of the die.