To provide a steel that has excellent toughness in an HAZ (Heat-Affected Zone) by large heat input welding with a heat input of 35 kJ/mm or more and has excellent joint CTOD properties.SOLUTION: A steel contains C, Si, Mn, P, S, Al, N, O, Ti, Zr, B and the balance being Fe and elemental impurities, with Insol.Zr: 0.0007-0.0040%, Sol.Zr: 0.0010% or less, and BF of 0.0030% or less. The steel has a carbon equivalent Ceq. of 0.30%-0.50%. The density of (Zr,B)-containing oxide particles having a circle-equivalent diameter of 0.5 μm or more and containing Zr of 5 mass% or more, B of 0.1 mass% or more, O of 1 mass% or more and Al2O3 of 50 mass% or less is 5-300/mm2 in number. Of the (Zr,B)-containing oxide particles, the ratio of (Mn,S)-coated (Zr,B)-containing oxide particles, in which both of Mn and S are deposited on the surface of the (Zr,B)-containing oxide particles, is 10% or more in number.SELECTED DRAWING: None