A steel material comprising a chemical composition consisting of, in mass %: C: 0.15 to 0.45%, Si: 0.10 to 1.0%, Mn: 0.10 to 0.8%, P: 0.050% or less, S: 0.010% or less, Al: 0.01 to 0.1%, N: 0.010% or less, Cr: 0.1 to 2.5%, Mo: 0.35 to 3.0%, Co: 0.05 to 2.0%, Ti: 0.003 to 0.040%, Nb: 0.003 to 0.050%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, V: 0 to 0.5%, B: 0 to 0.003%, W: 0 to 1.0%, Ca: 0 to 0.004%, Mg: 0 to 0.004%, and rare earth metal: 0 to 0.004%, with the balance being Fe and impurities, and satisfying Formulae and, wherein a prior-austenite grain diameter of a microstructure is less than 5 μm, as measured according to the method described in the description at a center of a thickness of the steel material, a block diameter of the microstructure is less than 2 μm at a center of a thickness of the steel material, wherein the block diameter is determined based on a crystal orientation map obtained by FESEM-EBSP measurement performed on 50 μm × 50 μm visual fields with 0.1 μm pitches therebetween, taking a Kikuchi pattern from the EBSP measurement, identifying from the Kikuchi pattern an orientation of aFe, determining a crystal orientation figure based on the orientation of aFe, determining from the crystal orientation figure zones surrounded by orientation differences with adjacent crystals of 15° or more, defining as one grain in a block a zone surrounded by orientation differences of 15° or more, determining for each block a circle equivalent diameter from its area, and calculating an average value of circle equivalent diameters in the visual fields and determining the average as the block diameter, and wherein a fracture toughness value K issc is 35 MPaVm or more, wherein the fracture toughness value K issc is measured using the method described in the description, the microstructure contains a total of 90% by volume or more of tempered martensite and tempered bainite: C + Mn / 6 + Cr + Mo + V / 5 + Cu + Ni / 15 − Co / 6 + α ≥ 0.70 3 C + Mo + 3 Co / 3 Mn + Cr ≥ 1.0 Effective B = B − 11 N − Ti / 3.4 / 14 where α in Formula is 0.250 when the effective B defined by Formula, in mass%, is 0.0003% or more, or zero when the effective B is less than 0.0003%, and symbols of elements in Formula to Formula are to be substituted by contents of corresponding elements in mass%. C: 0.15 to 0.45%, Si: 0.10 to 1.0%, Mn: 0.10 to 0.8%, P: 0.050% or less, S: 0.010% or less, Al: 0.01 to 0.1%, N: 0.010% or less, Cr: 0.1 to 2.5%, Mo: 0.35 to 3.0%, Co: 0.05 to 2.0%, Ti: 0.003 to 0.040%, Nb: 0.003 to 0.050%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, V: 0 to 0.5%, B: 0 to 0.003%, W: 0 to 1.0%, Ca: 0 to 0.004%, Mg: 0 to 0.004%, and rare earth metal: 0 to 0.004%, with the balance being Fe and impurities, and satisfying Formulae and, wherein a prior-austenite grain diameter of a microstructure is less than 5 μm, as measured according to the method described in the description at a center of a thickness of the steel material, a block diameter of the microstructure is less than 2 μm at a center of a thickness of the steel material, wherein the block diameter is determined based on a crystal orientation map obtained by FESEM-EBSP measurement performed on 50 μm × 50 μm visual fields with 0.1 μm pitches therebetween, taking a Kikuchi pattern from the EBSP measurement, identifying from the Kikuchi pattern an orientation of aFe, determining a crystal orientation figure based on the orientation of aFe, determining from the crystal orientation figure zones surrounded by orientation differences with adjacent crystals of 15° or more, defining as one grain in a block a zone surrounded by orientation differences of 15° or more, determining for each block a circle equivalent diameter from its area, and calculating an average value of circle equivalent diameters in the visual fields and determining the average as the block diameter, and wherein a fracture toughness value K issc is 35 MPaVm or more, wherein the fracture toughness value K issc is measured using the method described in the description, the microstructure contains a total of 90% by volume or more of tempered martensite and tempered bainite: C + Mn / 6 + Cr + Mo + V / 5 + Cu + Ni / 15 − Co / 6 + α ≥ 0.70 3 C + Mo + 3 Co / 3 Mn + Cr ≥ 1.0 Effective B = B − 11 N − Ti / 3.4 / 14 where α in Formula is 0.250 when the effective B defined by Formula, in mass%, is 0.0003% or more, or zero when the effective B is less than 0.0003%, and symbols of elements in Formula to Formula are to be substituted by contents of corresponding elements in mass%.