An H-section steel comprising, as a chemical composition, by mass%: C: 0.05% to 0.16%; Si: 0.01% to 0.50%; Mn: 0.70% to 2.00%; V: 0.01% to 0.20%; Al: 0.0001% to 0.10%; Ti: 0.003% to 0.030%; N: 0.0010% to 0.0200%; O: 0.0001% to 0.0100%; Mg: 0.0003% to 0.0050%; Ni: 0% to 0.50%; Cr: 0% to 0.50%; Cu: 0% to 0.50%; Mo: 0% to 0.30%; Nb: 0% to 0.010%; B: 0% to 0.0020%; Ca: 0% to 0.0050%; and a remainder of Fe and impurities, wherein a carbon equivalent C eq obtained by the following Equation 1 is 0.30% to 0.50%; a thickness of a flange is 100 mm to 150 mm; at a strength evaluation portion which is at a 1/6 position from a surface of the flange in a length direction and at a 1/4 position from the surface in a thickness direction, a fraction of bainite in a steel structure is 80% or more optionally, the remainder of the structure includes one or two or more of ferrite, pearlite and MA, and an average prior austenite grain size is 70 μm or more; and at a toughness evaluation portion which is at a 1/2 position from the surface of the flange in the length direction and at a 3/4 position from the surface of the flange in the thickness direction, the average prior austenite grain size in a steel structure is 200 μm or less wherein, at the strength evaluation portion at room temperature, a yield strength or 0.2% proof stress is 450 MPa or more and 550 MPa or less, and a tensile strength is 550 MPa or more and 680 MPa or less wherein the measurement is performed according to JIS Z 2241; and at the toughness evaluation portion, a Charpy absorbed energy at a test temperature of 21°C is 100 J or more wherein the measurement is performed according to JIS Z 2242; the total number density of Mg-containing oxides having an equivalent circle diameter of 0.005 μm to 0.5 μm at the toughness evaluation portion measured by using a transmission electron microscope is 100 pieces/mm 2 to 5000 pieces/mm 2; C eq = C + Mn / 6 + Cr + Mo + V / 5 + Ni + Cu / 15 here, C, Mn, Cr, Mo, V, Ni, and Cu represent the amount of each element contained by mass% and the amount of an element not contained is 0. C: 0.05% to 0.16%; Si: 0.01% to 0.50%; Mn: 0.70% to 2.00%; V: 0.01% to 0.20%; Al: 0.0001% to 0.10%; Ti: 0.003% to 0.030%; N: 0.0010% to 0.0200%; O: 0.0001% to 0.0100%; Mg: 0.0003% to 0.0050%; Ni: 0% to 0.50%; Cr: 0% to 0.50%; Cu: 0% to 0.50%; Mo: 0% to 0.30%; Nb: 0% to 0.010%; B: 0% to 0.0020%; Ca: 0% to 0.0050%; and a remainder of Fe and impurities, wherein a carbon equivalent C eq obtained by the following Equation 1 is 0.30% to 0.50%; a thickness of a flange is 100 mm to 150 mm; at a strength evaluation portion which is at a 1/6 position from a surface of the flange in a length direction and at a 1/4 position from the surface in a thickness direction, a fraction of bainite in a steel structure is 80% or more optionally, the remainder of the structure includes one or two or more of ferrite, pearlite and MA, and an average prior austenite grain size is 70 μm or more; and at a toughness evaluation portion which is at a 1/2 position from the surface of the flange in the length direction and at a 3/4 position from the surface of the flange in the thickness direction, the average prior austenite grain size in a steel structure is 200 μm or less wherein, at the strength evaluation portion at room temperature, a yield strength or 0.2% proof stress is 450 MPa or more and 550 MPa or less, and a tensile strength is 550 MPa or more and 680 MPa or less wherein the measurement is performed according to JIS Z 2241; and at the toughness evaluation portion, a Charpy absorbed energy at a test temperature of 21°C is 100 J or more wherein the measurement is performed according to JIS Z 2242; the total number density of Mg-containing oxides having an equivalent circle diameter of 0.005 μm to 0.5 μm at the toughness evaluation portion measured by using a transmission electron microscope is 100 pieces/mm 2 to 5000 pieces/mm 2; C eq = C + Mn / 6 + Cr + Mo + V / 5 + Ni + Cu / 15 here, C, Mn, Cr, Mo, V, Ni, and Cu represent the amount of each element contained by mass% and the amount of an element not contained is 0.