A method includes: forming either one or both of convex and concave bead portions at surface portions corresponding to sites of a side wall portion on both sides of a convex curve site and to sites of a side wall portion on both sides of a concave curve site; and squashing the bead portions, wherein the bead portions formed on both sides of the convex curve site are tilted such that an end part of the bead portion positioned on the convex curve site side is farther from a baseline and the other end part is closer to the baseline; and the bead portions formed on both sides of the concave curve site are tilted such that an end part of the bead portion positioned on the concave curve site side is closer to the baseline and the other end part is farther from the baseline.