Abstract: A void having a side peripheral surface and a bottom part is machined in a rotationally symmetrical shape spanning the end surface of a first steel member and the end surface of a second steel member; in a state in which a pressing force is applied to a contact area between the tip part of a joining metal and the bottom part of the void, the joining metal is rotated around a rotation axis and friction is created; the material structure around a rotational friction surface is joined using friction heat caused by the friction and molten metal is generated; a gap between a side peripheral surface of the joining metal and the side peripheral surface of the void is filled with the liquefied molten metal; and the first steel member and the second steel member are joined via the joining metal through integration with the structure near the gap.