A structural member 10 includes a closed-cross-section structure having a hat member 1 and a closing plate 2, and a reinforcing member (supplementary strengthening member ("SSM")) 4. The hat member 1 includes two side walls 11 and 12, each of which includes a high-strength portion 11A, 12A with a yield strength not lower than 500 MPa and low-strength portions 11B, 12B with a yield strength of 60 to 85 % of the yield strength of the high-strength portion. As determined along the direction of extension of the ridges 123 of the hat member, both ends of the reinforcing member 4 overlap the high-strength portions 11A and 12B of the side walls 11 and 12. Adjacent to at least one of both ends of the reinforcing member 4, the border between the high-strength portions 11A and 12A and the low-strength portions 11B and 12B is positioned in a region starting at a position 1/3, and ending at a position 3/2, of the height H of the side wall 11 distant from that end of the reinforcing member 4 as determined along the direction of extension of the ridges 123 away from the reinforcing member 4.