A plurality of beads (2) extending in the vehicle longitudinal direction are formed parallel to each other lined up in the vehicle width direction on an inner panel (1). Accordingly, an upper flange (5), a vertical wall (4), a lower flange (3), and a vertical wall (4) are formed connected in this order at a cross-section in the vehicle width direction. The vertical wall (4) includes a lower vertical wall (4a) and an upper vertical wall (4b), and there is a bend line (4c) at the boundary between the two which is bent. The inclination angle of the upper vertical wall (4b) is between 30° and 60° inclusive, which is greater than the inclination angle of the lower vertical wall (4a). The bend line (4c) is positioned more than ½ way up between the lower flange (3) and the upper flange (5) so as to be closer to the upper flange (5) than the lower flange (3).