A bumper beam (121; 131; 141; 151), which is configured to be mounted on the front or rear of a vehicle body and to extend along a reference line which is substantially matched to a width direction, comprising: a beam body formed of a metallic plate, and extending along the reference line; and a metallic reinforcing plate (4; 4, 5; 14, 15; 24) extending along the reference line, and attached to said beam body, wherein in at least one cross section perpendicular to the reference line including a cross section in a central part of said bumper beam (121; 131; 141; 151) in the width direction, said beam body includes a reference wall extending linearly on a collision surface side of said beam body; and a pair of swelling parts (2s, 2t) each swelling from the reference wall in a direction toward a vehicle body side of said beam body while enclosing an internal space, each of said swelling parts (2s, 2t) having an outer wall part (2a, 2c) extending in a direction intersecting the reference wall and an inner wall part (2e, 2k) extending in a direction intersecting the reference wall at a position closer than the position of said outer wall part (2a, 2c) to the other swelling part (2s, 2t), each outer wall part (2a, 2c) and each inner wall part (2e, 2k) being aligned along the reference line; and a said reinforcing plate (4; 4, 5; 14, 15; 24) is attached to both said outer wall parts (2a, 2c) and/or both said inner wall parts (2e, 2k) in parallel thereto, characterized in that said beam body and said reinforcing plate (4; 4, 5; 14, 15; 24) satisfy the following condition: E st / ρ st 3 < E 3 / ρ 3 3 E st: Young's modulus of said beam body ρ st: Density of said beam body E 3: Young's modulus of said reinforcing plate (4; 4, 5; 14, 15; 24) ρ 3: Density of said reinforcing plate (4; 4, 5; 14, 15; 24). a beam body formed of a metallic plate, and extending along the reference line; and a metallic reinforcing plate (4; 4, 5; 14, 15; 24) extending along the reference line, and attached to said beam body, wherein in at least one cross section perpendicular to the reference line including a cross section in a central part of said bumper beam (121; 131; 141; 151) in the width direction, said beam body includes a reference wall extending linearly on a collision surface side of said beam body; and a pair of swelling parts (2s, 2t) each swelling from the reference wall in a direction toward a vehicle body side of said beam body while enclosing an internal space, each of said swelling parts (2s, 2t) having an outer wall part (2a, 2c) extending in a direction intersecting the reference wall and an inner wall part (2e, 2k) extending in a direction intersecting the reference wall at a position closer than the position of said outer wall part (2a, 2c) to the other swelling part (2s, 2t), each outer wall part (2a, 2c) and each inner wall part (2e, 2k) being aligned along the reference line; and a said reinforcing plate (4; 4, 5; 14, 15; 24) is attached to both said outer wall parts (2a, 2c) and/or both said inner wall parts (2e, 2k) in parallel thereto, characterized in that said beam body and said reinforcing plate (4; 4, 5; 14, 15; 24) satisfy the following condition: E st / ρ st 3 < E 3 / ρ 3 3 E st: Young's modulus of said beam body ρ st: Density of said beam body E 3: Young's modulus of said reinforcing plate (4; 4, 5; 14, 15; 24) ρ 3: Density of said reinforcing plate (4; 4, 5; 14, 15; 24). E st: Young's modulus of said beam body ρ st: Density of said beam body E 3: Young's modulus of said reinforcing plate (4; 4, 5; 14, 15; 24) ρ 3: Density of said reinforcing plate (4; 4, 5; 14, 15; 24).