An assembled body including an impact-absorbing member and a structural member, the impact absorbing member comprising a partial tubular body having a polygonal shape with one open side in a cross-section perpendicular to a longitudinal direction of the partial tubular body and a flange portion; and the structural member having a plate-like shape and being placed so as to face the flange portion of the impact-absorbing member and to close the open side of the polygonal shape in the cross-section of the partial tubular body, wherein the partial tubular body includes: a plurality of flat portions; and a plurality of corner portions provided between the flat portions, wherein: at least one of the plurality of flat portions has a thermally-treated portion formed by applying a thermal treatment with a laser light so as to deform at an angle of not less than 0.1 degrees and not more than 3.0 degrees; each of the flat portions is separated into a plurality of quadrilateral partial flat portions adjacent to each other when the partial tubular body is divided by sections perpendicular to the longitudinal direction arranged in the longitudinal direction at L/2 intervals from a vertical section located at a predetermined position along the longitudinal direction from an end portion in the longitudinal direction of the partial tubular body, where L is an average side length of all the sides constituting the polygonal shape with one open side at the end portion; the thermally-treated portion is formed along at least one diagonal direction of at least one quadrilateral partial flat portion of the plurality of quadrilateral partial flat portions; and the thermally-treated portion is provided to at least two quadrilateral partial flat portions adjacent to each other of the plurality of quadrilateral partial flat portions so as to be symmetric with respect to a boundary line between the adjacent quadrilateral partial flat portions. a plurality of flat portions; and a plurality of corner portions provided between the flat portions, wherein: at least one of the plurality of flat portions has a thermally-treated portion formed by applying a thermal treatment with a laser light so as to deform at an angle of not less than 0.1 degrees and not more than 3.0 degrees; each of the flat portions is separated into a plurality of quadrilateral partial flat portions adjacent to each other when the partial tubular body is divided by sections perpendicular to the longitudinal direction arranged in the longitudinal direction at L/2 intervals from a vertical section located at a predetermined position along the longitudinal direction from an end portion in the longitudinal direction of the partial tubular body, where L is an average side length of all the sides constituting the polygonal shape with one open side at the end portion; the thermally-treated portion is formed along at least one diagonal direction of at least one quadrilateral partial flat portion of the plurality of quadrilateral partial flat portions; and the thermally-treated portion is provided to at least two quadrilateral partial flat portions adjacent to each other of the plurality of quadrilateral partial flat portions so as to be symmetric with respect to a boundary line between the adjacent quadrilateral partial flat portions. at least one of the plurality of flat portions has a thermally-treated portion formed by applying a thermal treatment with a laser light so as to deform at an angle of not less than 0.1 degrees and not more than 3.0 degrees; each of the flat portions is separated into a plurality of quadrilateral partial flat portions adjacent to each other when the partial tubular body is divided by sections perpendicular to the longitudinal direction arranged in the longitudinal direction at L/2 intervals from a vertical section located at a predetermined position along the longitudinal direction from an end portion in the longitudinal direction of the partial tubular body, where L is an average side length of all the sides constituting the polygonal shape with one open side at the end portion; the thermally-treated portion is formed along at least one diagonal direction of at least one quadrilateral partial flat portion of the plurality of quadrilateral partial flat portions; and the thermally-treated portion is provided to at least two quadrilateral partial flat portions adjacent to each other of the plurality of quadrilateral partial flat portions so as to be symmetric with respect to a boundary line between the adjacent quadrilateral partial flat portions.