To improve support rigidity in one in which a suspension bracket is mounted to a lower side of a closed section formed by a floor pannel and a side member by recessedly forming a bolt mounting surface at a bottom wall of the side member.

In a rear side member 1 of a substantially hat shape in section which is a skeleton-like member in the forward and rearward direction of a vehicle, a lower surface of a rear floor pannel 2 is joined to an upper surface of flanges 1a, 1a at an upper edge of the member 1 and thereby a closed sectional part 4 is formed. To the part 4, a suspension bolt 5 to be used in mounting a suspension bracket is fixed. In mounting the bolt 5, a mounting surface 6 to which a mounting part 10 of the bolt 5 is recessedly formed substantially in parallel at a bottom wall 1a of the member 1. A reinforcement 8 is provided so that it is fitted in the member 1, and an outer wall 8a thereof is joined to an inner wall of the member 1 to thereby form a closed sectional part 9.