This is a method of forming a fracture start portion of a ductile metal part on an inner circumferential face of a through hole by irradiating laser to an opposing position of the inner circumferential face of the thorough hole of the ductile metal part having a predetermined through hole and by forming a large number of recess portions separated with a predetermined distance at a predetermined interval from one opening to the other opening of the through hole, in which a recess part is formed, instead of by focusing the laser exactly onto the inner circumferential face of the through hole, by irradiating it onto the inner circumferential face of the through hole while defocusing from the inner circumferential face of the through hole by a predetermined amount as well as by supplying an assist gas to a position of the laser irradiation, and the laser is moved linearly at a predetermined speed from one side opening to the other side opening on the inner circumferential face of the through hole while irradiating the laser onto the inner circumferential face of the through hole at a predetermined pulse.