A steel plate comprising, as a chemical composition, by mass%: C: 0.16% to 0.20%; Si: 0.50% to 1.00%; Mn: 0.90% to 1.50%; P: 0.010% or less; S: 0.0020% or less; Cu: 0% to 0.40%; Ni: 0.20% to 1.00%; Cr: 0.60% to 0.99%; Mo: 0.60% to 1.00%; V: 0% to 0.050%; Al: 0.050% to 0.085%; N: 0.0020% to 0.0070%; B: 0.0005% to 0.0020%; Nb: 0% to 0.050%; Ti: 0% to 0.020%; Ca: 0% to 0.0030%; Mg: 0% to 0.0030%; REM: 0% to 0.0030%; and a remainder including Fe and impurities, wherein a total area ratio of martensite and bainite in a thickness middle portion is 99% or more the middle portion being the region between a plane at a depth of 3/8 of a plate thickness of the steel plate from a rolled surface, which is the outermost surface of the steel plate, and a plane at a depth of 5/8 of the plate thickness of the steel plate from the rolled surface, an average value of a prior austenite grain size in the thickness middle portion is less than 80 μm, Ceq represented by Formula is 0.750% to 0.800%, Al × N is 2.0 × 10 -4 or more, Ti/N is 3.4 or less, a value f represented by Formula and a value g represented by Formula satisfy 4 × f/g ≥ 9.00, a -20°C Charpy absorbed energy measured in a C direction in the thickness middle portion, which is determined in accordance with the description, is 47 J or more, hardnesses of a surface layer and the thickness middle portion, which is determined in accordance with the description, are HB 350 or more, and a plate thickness of the steel plate is more than 200 mm, Ceq = C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V / 5: f = 4 × C + Si + 2 × Mn + Ni + 2 × Cr + 5 × Mo: g = 2 × Cr + 3 × Mo + 5 × V: where each element symbol described in each of the formulas means an amount of an element corresponding to the element symbol in unit mass%. C: 0.16% to 0.20%; Si: 0.50% to 1.00%; Mn: 0.90% to 1.50%; P: 0.010% or less; S: 0.0020% or less; Cu: 0% to 0.40%; Ni: 0.20% to 1.00%; Cr: 0.60% to 0.99%; Mo: 0.60% to 1.00%; V: 0% to 0.050%; Al: 0.050% to 0.085%; N: 0.0020% to 0.0070%; B: 0.0005% to 0.0020%; Nb: 0% to 0.050%; Ti: 0% to 0.020%; Ca: 0% to 0.0030%; Mg: 0% to 0.0030%; REM: 0% to 0.0030%; and a remainder including Fe and impurities, wherein a total area ratio of martensite and bainite in a thickness middle portion is 99% or more the middle portion being the region between a plane at a depth of 3/8 of a plate thickness of the steel plate from a rolled surface, which is the outermost surface of the steel plate, and a plane at a depth of 5/8 of the plate thickness of the steel plate from the rolled surface, an average value of a prior austenite grain size in the thickness middle portion is less than 80 μm, Ceq represented by Formula is 0.750% to 0.800%, Al × N is 2.0 × 10 -4 or more, Ti/N is 3.4 or less, a value f represented by Formula and a value g represented by Formula satisfy 4 × f/g ≥ 9.00, a -20°C Charpy absorbed energy measured in a C direction in the thickness middle portion, which is determined in accordance with the description, is 47 J or more, hardnesses of a surface layer and the thickness middle portion, which is determined in accordance with the description, are HB 350 or more, and a plate thickness of the steel plate is more than 200 mm, Ceq = C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V / 5: f = 4 × C + Si + 2 × Mn + Ni + 2 × Cr + 5 × Mo: g = 2 × Cr + 3 × Mo + 5 × V: where each element symbol described in each of the formulas means an amount of an element corresponding to the element symbol in unit mass%.