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 wails (11 and 12), each of which includes a high-strength portion (11A, 12A) with a yield strength not lower than 500 MFa 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).