A reinforcement member (1, 1A, 1B) for a vehicle, comprising: a cylindrical body section (100, 100A) formed by molding a metal pipe to have a continuously closed cross section intersecting with a longitudinal direction, wherein the cylindrical body section (100, 100A) has a first surface located inside a vehicle when mounted on the vehicle, a second surface separated from and facing the first surface, and a third surface (103a, 103b, 103c, 103d) connecting the first surface and the second surface to each other, the third surface (103a, 103b, 103c, 103d) has a fourth surface (121, 121A, 151, 151A) extending away from the second surface, and a fifth surface (122, 122A, 152, 152A) connecting the fourth surface (121, 121A, 151, 151A) and the first surface to each other, and in a sectional view when viewed in the longitudinal direction, in a case where a virtual line connecting a first intersection portion (181, 191) where the first surface and the third surface (103a, 103b, 103c, 103d) intersect with each other and a second intersection portion (182, 192) where the second surface and the third surface (103a, 103b, 103c, 103d) intersect with each other is set, a third intersection portion (183, 193) where the fourth surface (121, 121A, 151, 151A) and the fifth surface (122, 122A, 152, 152A) intersect with each other protrudes outward with respect to the virtual line, characterized in that the cylindrical body section (100, 100A) has a variable portion (196, 197) where a position of the third intersection portion (183, 193) in a facing direction of the first surface and the second surface varies along the longitudinal direction, as the variable portion (196, 197), the cylindrical body section (100, 100A) has a first variable portion where the position of the third intersection portion (183, 193) varies from the first surface side to the second surface side, as the first variable portion is oriented toward one side in the longitudinal direction, and a second variable portion where the position of the third intersection portion (183, 193) varies from the second surface side to the first surface side side, as the second variable portion is oriented toward the one side in the longitudinal direction, and in at least a portion of the cylindrical body section (100, 100A) in the longitudinal direction, the first variable portion and the second variable portion are alternately formed along the longitudinal direction in a predetermined pattern having repeated shapes. a cylindrical body section (100, 100A) formed by molding a metal pipe to have a continuously closed cross section intersecting with a longitudinal direction, wherein the cylindrical body section (100, 100A) has a first surface located inside a vehicle when mounted on the vehicle, a second surface separated from and facing the first surface, and a third surface (103a, 103b, 103c, 103d) connecting the first surface and the second surface to each other, the third surface (103a, 103b, 103c, 103d) has a fourth surface (121, 121A, 151, 151A) extending away from the second surface, and a fifth surface (122, 122A, 152, 152A) connecting the fourth surface (121, 121A, 151, 151A) and the first surface to each other, and in a sectional view when viewed in the longitudinal direction, in a case where a virtual line connecting a first intersection portion (181, 191) where the first surface and the third surface (103a, 103b, 103c, 103d) intersect with each other and a second intersection portion (182, 192) where the second surface and the third surface (103a, 103b, 103c, 103d) intersect with each other is set, a third intersection portion (183, 193) where the fourth surface (121, 121A, 151, 151A) and the fifth surface (122, 122A, 152, 152A) intersect with each other protrudes outward with respect to the virtual line, characterized in that the cylindrical body section (100, 100A) has a variable portion (196, 197) where a position of the third intersection portion (183, 193) in a facing direction of the first surface and the second surface varies along the longitudinal direction, as the variable portion (196, 197), the cylindrical body section (100, 100A) has a first variable portion where the position of the third intersection portion (183, 193) varies from the first surface side to the second surface side, as the first variable portion is oriented toward one side in the longitudinal direction, and a second variable portion where the position of the third intersection portion (183, 193) varies from the second surface side to the first surface side side, as the second variable portion is oriented toward the one side in the longitudinal direction, and a first variable portion where the position of the third intersection portion (183, 193) varies from the first surface side to the second surface side, as the first variable portion is oriented toward one side in the longitudinal direction, and a second variable portion where the position of the third intersection portion (183, 193) varies from the second surface side to the first surface side side, as the second variable portion is oriented toward the one side in the longitudinal direction, and in at least a portion of the cylindrical body section (100, 100A) in the longitudinal direction, the first variable portion and the second variable portion are alternately formed along the longitudinal direction in a predetermined pattern having repeated shapes.