A steel sheet comprising, as a chemical composition, by mass%: C: 0.15% to 0.40%; Si: 0.01% to 2.00%; Mn: 0.10% to 4,00%; Al: 0.005% to 1.500%; P: 0.001% to 0.100%; S: 0.0005% to 0.0100%; N: 0.0005% to 0.0100%; Ti: 0% to 0.200%; Mo: 0% to 0.300%; Nb: 0% to 0.200%; Cr: 0% to 4,000%; B: 0% to 0.0050%; V: 0% to 0.300%; Ni: 0% to 4.00%; Cu: 0% to 4,00%; W: 0% to 2,00%; Ca: 0 % to 0.0100%; Ce: 0% to 0.0100%; Mg: 0% to 0.0100%; Zr: 0% to 0.0100%; La: 0% to 0.0100%; REM other than Ce and La: 0%to 0.0100%; Sn: 0% to 1.000%; Sb: 0% to 0.200%; and a remainder: Fe and impurities, wherein a microstructure at a 1/4 thickness which is a range between a 1/8 thickness position in a sheet thickness direction from a surface of the steel sheet and a 3/8 thickness position in the sheet thickness direction from the surface includes, by area ratio, ferrite: 0% to 10%, retained austenite: 0% to 10%, upper bainite: 0% to 10%, martensite: 70% to 100%, and pearlite: 0% to 5%, an area ratio of plate martensite contained in the martensite is 10% to 35% with respect to an area of an entire structure of the steel sheet, an average grain size of prior austenite grains is 2.0 μm to 10.0 μm, and a maximum diameter of the prior austenite grains is 20.0 μm or less, an amount of solid solution C in the martensite is 0.20 mass% or less, an average carbide size in the martensite is 0.25 μm or less, a crystal orientation difference between the plate martensite and another martensite adjacent to the plate martensite in the same prior austenite grain is 10.0° or less, and a P concentration at grain boundaries of the prior austenite grains is 4.0 at% or less, wherein the microstructure, the solid solution C in the martensite, and the P concentration at the grain boundaries are determined as outlined in the description. C: 0.15% to 0.40%; Si: 0.01% to 2.00%; Mn: 0.10% to 4,00%; Al: 0.005% to 1.500%; P: 0.001% to 0.100%; S: 0.0005% to 0.0100%; N: 0.0005% to 0.0100%; Ti: 0% to 0.200%; Mo: 0% to 0.300%; Nb: 0% to 0.200%; Cr: 0% to 4,000%; B: 0% to 0.0050%; V: 0% to 0.300%; Ni: 0% to 4.00%; Cu: 0% to 4,00%; W: 0% to 2,00%; Ca: 0 % to 0.0100%; Ce: 0% to 0.0100%; Mg: 0% to 0.0100%; Zr: 0% to 0.0100%; La: 0% to 0.0100%; REM other than Ce and La: 0%to 0.0100%; Sn: 0% to 1.000%; Sb: 0% to 0.200%; and a remainder: Fe and impurities, wherein a microstructure at a 1/4 thickness which is a range between a 1/8 thickness position in a sheet thickness direction from a surface of the steel sheet and a 3/8 thickness position in the sheet thickness direction from the surface includes, by area ratio, ferrite: 0% to 10%, retained austenite: 0% to 10%, upper bainite: 0% to 10%, martensite: 70% to 100%, and pearlite: 0% to 5%, ferrite: 0% to 10%, retained austenite: 0% to 10%, upper bainite: 0% to 10%, martensite: 70% to 100%, and pearlite: 0% to 5%, an area ratio of plate martensite contained in the martensite is 10% to 35% with respect to an area of an entire structure of the steel sheet, an average grain size of prior austenite grains is 2.0 μm to 10.0 μm, and a maximum diameter of the prior austenite grains is 20.0 μm or less, an amount of solid solution C in the martensite is 0.20 mass% or less, an average carbide size in the martensite is 0.25 μm or less, a crystal orientation difference between the plate martensite and another martensite adjacent to the plate martensite in the same prior austenite grain is 10.0° or less, and a P concentration at grain boundaries of the prior austenite grains is 4.0 at% or less, wherein the microstructure, the solid solution C in the martensite, and the P concentration at the grain boundaries are determined as outlined in the description. <EOS>