A steel plate comprising, as a chemical composition, by mass%: C: 0.20% to 0.35%; Si: more than 1.00% to 2.00%; Mn: 0.60% to 2.00%; Cr: 0.10% to 2.00%; Mo: 0.05% to 1.00%; Al: 0.010% to 0.100%; N: 0.0020% to 0.0100%; B: 0.0003% to 0.0020%; P: 0.0200% or less; S: less than 0.0100%; Cu: 0% to 0.500%; Ni: 0% to 1.00%; Nb: 0% to 0.050%; V: 0% to 0.120%; Ti: 0% to 0.025%; Ca: 0% to 0.050%; Mg: 0% to 0.050%; REM: 0% to 0.100%; and a remainder consisting of Fe and impurities, wherein an index Q obtained by Equation is 0.00 or more, a carbon equivalent Ceq (%) obtained by Equation is less than 0.800%, a ratio ΔHv/Hvs (%) of a difference between a hardness at a surface layer portion Hvs and a hardness at a thickness center portion Hvc to the hardness at the surface layer portion Hvs at a room temperature is 15.0% or less and the hardness at the surface layer portion Hvs at a room temperature is 400 or more in terms of Vickers hardness HV5 and has been measured as explained in the description, a steel thickness T is 40 mm or more, and wherein the ratio ΔHv/Hvs (%) is expressed by Equation (b) Q = 0.18 − 1.3 logT + 0.75 2.7 × C + Mn + 0.45 × Ni + 0.8 × Cr + 2 × Mo Ceq % = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 4 Δ ⁢ Hv / Hvs % = 100 × Hvs − Hvc / Hvs where the index Q of Equation is calculated by substituting a numerical value of the steel thickness T (mm), a numerical value of an amount of each element X in terms of mass% and 0 in a case where the element X is not contained, the carbon equivalent Ceq (%) of Equation is calculated by substituting a numerical value of an amount of each element X in terms of mass% and 0 in a case where the element X is not contained. C: 0.20% to 0.35%; Si: more than 1.00% to 2.00%; Mn: 0.60% to 2.00%; Cr: 0.10% to 2.00%; Mo: 0.05% to 1.00%; Al: 0.010% to 0.100%; N: 0.0020% to 0.0100%; B: 0.0003% to 0.0020%; P: 0.0200% or less; S: less than 0.0100%; Cu: 0% to 0.500%; Ni: 0% to 1.00%; Nb: 0% to 0.050%; V: 0% to 0.120%; Ti: 0% to 0.025%; Ca: 0% to 0.050%; Mg: 0% to 0.050%; REM: 0% to 0.100%; and a remainder consisting of Fe and impurities, wherein an index Q obtained by Equation is 0.00 or more, a carbon equivalent Ceq (%) obtained by Equation is less than 0.800%, a ratio ΔHv/Hvs (%) of a difference between a hardness at a surface layer portion Hvs and a hardness at a thickness center portion Hvc to the hardness at the surface layer portion Hvs at a room temperature is 15.0% or less and the hardness at the surface layer portion Hvs at a room temperature is 400 or more in terms of Vickers hardness HV5 and has been measured as explained in the description, a steel thickness T is 40 mm or more, and wherein the ratio ΔHv/Hvs (%) is expressed by Equation (b) Q = 0.18 − 1.3 logT + 0.75 2.7 × C + Mn + 0.45 × Ni + 0.8 × Cr + 2 × Mo Ceq % = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 4 Δ ⁢ Hv / Hvs % = 100 × Hvs − Hvc / Hvs where the index Q of Equation is calculated by substituting a numerical value of the steel thickness T (mm), a numerical value of an amount of each element X in terms of mass% and 0 in a case where the element X is not contained, the carbon equivalent Ceq (%) of Equation is calculated by substituting a numerical value of an amount of each element X in terms of mass% and 0 in a case where the element X is not contained.