Side sill includes a side sill body formed of a steel plate, and a side sill reinforcing member formed by extruding an aluminum alloy and provided inside the side sill body. Reinforcing bracket formed by casting an aluminum alloy covers the outer surface of the side sill body. The reinforcing bracket has protruding portions projecting, through through-holes formed in the side sill body, generally toward the interior of the side sill to be fixed to portions of the side sill reinforcing member, so that the side sill reinforcing member is restrainedly held by the reinforcing bracket.