Abstract: This manufacturing method comprises a first step in which a molten aluminum alloy is cast into casting dies into which a ball 8 forming a ball portion 1a of a ball-shank 1 in corporation with a shank 1b connected to the portion 1a is inserted as a core so that a holder 2 covering the ball portion 1a is formed leaving the connected portion of the shank 1b as it is; a second step in which the aluminum alloy for forming the holder 2 is subjected to a solution heat treatment; a third step in which the shank 1b is brought into pressure contact with the ball 8 exposed from the holder 2, electrodes 10a and 10b are held in contact with the shank 1b and the holder 2, respectively, and the ball 8 and the shank 1b are projection-welded by turning on electricity between electrodes to thereby form the ball-shank 1; and a fourth step in which an external force is applied to the holder 2 or the ball-shank 1 to thereby provide a gap between the ball 8 and the holder 2.