A torsion portion extending in a vehicle width direction and having a first cross-sectional area, shoulder portions located at both ends of the torsion portion and having a second cross-sectional area larger than the first cross-sectional area and arm portions extending from the shoulder portions in a front and back direction of a vehicle, respectively and having the first cross-sectional area, so that even when a hollow member is used for weight saving, can be realized by design and manufacturing method facilitating making uniform principal stress as in a solid member.