PURPOSE: To improve reinforcement effect by forming a large closed section by an upper member and a lower member of reinforcement and by forming a small closed section between a joining flange and an apron side member.

CONSTITUTION: A tire house arch 5 is formed in a wheel apron side member 2b, and a reinforcement 6 is arranged outside the side member 26. A lower member 6a of the reinforcement 6 has a joining flange 8 and a horizontal part 7, an upper member 6a has an upper joining flange 9 and a lower joining flange 10. The flange 8 of a lower member 6a has a channel-shaped part 8a, the part 8a forms a small closed section structure with the member 2b, at the time the joining flange 8 is welded to the wheel apron side member 2b. The upper member 6b forms a closed section structure, with the joining flange 9 welded to a flange 11 of the member 2b and with the flange 10 welded to outside part of the member 6a.