A structural member comprising: a closed-cross-section structure including at least one closing plate and a hat member; wherein the hat member includes: a top-plate portion; two first ridges, each positioned on a respective one of both ends of the top-plate portion; two flanges joined to the closing plate; two second ridges, each positioned on an end of a respective one of the two flanges; and two side walls (11, 12), each located between a respective one of the first ridges and a respective one of the second ridges, heights of the two side walls (11, 12) as measured in a direction perpendicular to the top-plate portion being denoted by H1 and H2, characterised by further comprising a reinforcing member attached to the closed-cross-section structure, and in that: each of the two side walls (11, 12) includes: a high-strength portion having a yield strength not lower than 500 MPa; and a low-strength portion adjacent to the high-strength portion in an extending direction of the first ridge, the low-strength portion having a yield strength of 60 to 85 % of the yield strength of the high-strength portion, and the reinforcing member is adjacent to at least a part of the top-plate portion of the hat member, both ends of the reinforcing member as determined along the direction of extension of the first ridges overlap the high-strength portions of the side walls (11, 12), and, a boundary between the high-strength portion and the low-strength portion is from 1/3 of H1 to 3/2 of H1 and from 1/3 of H2 to 3/2 of H2 away from at least one of both ends of the reinforcing member in the extending direction of the first ridge. a closed-cross-section structure including at least one closing plate and a hat member; wherein the hat member includes: a top-plate portion; two first ridges, each positioned on a respective one of both ends of the top-plate portion; two flanges joined to the closing plate; two second ridges, each positioned on an end of a respective one of the two flanges; and two side walls (11, 12), each located between a respective one of the first ridges and a respective one of the second ridges, heights of the two side walls (11, 12) as measured in a direction perpendicular to the top-plate portion being denoted by H1 and H2, characterised by further comprising a reinforcing member attached to the closed-cross-section structure, and in that: a top-plate portion; two first ridges, each positioned on a respective one of both ends of the top-plate portion; two flanges joined to the closing plate; two second ridges, each positioned on an end of a respective one of the two flanges; and two side walls (11, 12), each located between a respective one of the first ridges and a respective one of the second ridges, heights of the two side walls (11, 12) as measured in a direction perpendicular to the top-plate portion being denoted by H1 and H2, characterised by further comprising a reinforcing member attached to the closed-cross-section structure, and in that: each of the two side walls (11, 12) includes: a high-strength portion having a yield strength not lower than 500 MPa; and a low-strength portion adjacent to the high-strength portion in an extending direction of the first ridge, the low-strength portion having a yield strength of 60 to 85 % of the yield strength of the high-strength portion, and the reinforcing member is adjacent to at least a part of the top-plate portion of the hat member, both ends of the reinforcing member as determined along the direction of extension of the first ridges overlap the high-strength portions of the side walls (11, 12), and, a boundary between the high-strength portion and the low-strength portion is from 1/3 of H1 to 3/2 of H1 and from 1/3 of H2 to 3/2 of H2 away from at least one of both ends of the reinforcing member in the extending direction of the first ridge. a high-strength portion having a yield strength not lower than 500 MPa; and a low-strength portion adjacent to the high-strength portion in an extending direction of the first ridge, the low-strength portion having a yield strength of 60 to 85 % of the yield strength of the high-strength portion, and the reinforcing member is adjacent to at least a part of the top-plate portion of the hat member, both ends of the reinforcing member as determined along the direction of extension of the first ridges overlap the high-strength portions of the side walls (11, 12), and, a boundary between the high-strength portion and the low-strength portion is from 1/3 of H1 to 3/2 of H1 and from 1/3 of H2 to 3/2 of H2 away from at least one of both ends of the reinforcing member in the extending direction of the first ridge.