A steel sheet comprising: C: 0.35 to 0.50% by mass, Si: 2.1 to 2.8% by mass, Mn: 1.2 to 1.8% by mass, P: 0.05% by mass or less, S: 0.01% by mass or less, Al: 0.01 to 0.1% by mass, and optionally satisfying one or more of the following (a) to (c): (a) further comprising one or more elements selected from the group consisting of: V: 0.001 to 0.05% by mass, Nb: 0.001 to 0.05% by mass, Ti: 0.001 to 0.05% by mass, Zr: 0.001 to 0.05% by mass, and Hf: 0.001 to 0.05% by mass; (b) further comprising one or more elements selected from the group consisting of: Cr: 0.001 to 0.50% by mass, Mo: 0.001 to 0.50% by mass, Ni: 0.001 to 0.50% by mass, Cu: 0.001 to 0.50% by mass, and B: 0.0001 to 0.0050% by mass; and (c) further comprising one or more elements selected from the group consisting of: Ca: 0.0001 to 0.0010% by mass, Mg: 0.0001 to 0.0010% by mass, Li: 0.0001 to 0.0010% by mass, and REM: 0.0001 to 0.0010% by mass; with the balance consisting of iron and inevitable impurities, wherein a total area ratio of bainite, bainitic ferrite, martensite, residual austenite, and a martensite-austenite mixed structure, measured in accordance with the method described in the description, is 95% or more and 100% or less, a total area ratio of ferrite and pearlite, measured in accordance with the method described in the description, is less than 5%, an area ratio of the martensite-austenite mixed structure, measured in accordance with the method described in the description, is 5% or more and 30% or less, an average section length of the martensite-austenite mixed structure, measured in accordance with the method described in the description, is 0.32 μm or less, and a proportion of an area of a region in ferrite, bainitic ferrite, and martensite where no cementite is present to the total area of ferrite, bainitic ferrite, and martensite, measured in accordance with the method described in the description, is 3.0% or more and 5.0% or less. C: 0.35 to 0.50% by mass, Si: 2.1 to 2.8% by mass, Mn: 1.2 to 1.8% by mass, P: 0.05% by mass or less, S: 0.01% by mass or less, Al: 0.01 to 0.1% by mass, and optionally satisfying one or more of the following (a) to (c): (a) further comprising one or more elements selected from the group consisting of: V: 0.001 to 0.05% by mass, Nb: 0.001 to 0.05% by mass, Ti: 0.001 to 0.05% by mass, Zr: 0.001 to 0.05% by mass, and Hf: 0.001 to 0.05% by mass; (b) further comprising one or more elements selected from the group consisting of: Cr: 0.001 to 0.50% by mass, Mo: 0.001 to 0.50% by mass, Ni: 0.001 to 0.50% by mass, Cu: 0.001 to 0.50% by mass, and B: 0.0001 to 0.0050% by mass; and (c) further comprising one or more elements selected from the group consisting of: Ca: 0.0001 to 0.0010% by mass, Mg: 0.0001 to 0.0010% by mass, Li: 0.0001 to 0.0010% by mass, and REM: 0.0001 to 0.0010% by mass; (a) further comprising one or more elements selected from the group consisting of: V: 0.001 to 0.05% by mass, Nb: 0.001 to 0.05% by mass, Ti: 0.001 to 0.05% by mass, Zr: 0.001 to 0.05% by mass, and Hf: 0.001 to 0.05% by mass; V: 0.001 to 0.05% by mass, Nb: 0.001 to 0.05% by mass, Ti: 0.001 to 0.05% by mass, Zr: 0.001 to 0.05% by mass, and Hf: 0.001 to 0.05% by mass; (b) further comprising one or more elements selected from the group consisting of: Cr: 0.001 to 0.50% by mass, Mo: 0.001 to 0.50% by mass, Ni: 0.001 to 0.50% by mass, Cu: 0.001 to 0.50% by mass, and B: 0.0001 to 0.0050% by mass; and Cr: 0.001 to 0.50% by mass, Mo: 0.001 to 0.50% by mass, Ni: 0.001 to 0.50% by mass, Cu: 0.001 to 0.50% by mass, and B: 0.0001 to 0.0050% by mass; and (c) further comprising one or more elements selected from the group consisting of: Ca: 0.0001 to 0.0010% by mass, Mg: 0.0001 to 0.0010% by mass, Li: 0.0001 to 0.0010% by mass, and REM: 0.0001 to 0.0010% by mass; Ca: 0.0001 to 0.0010% by mass, Mg: 0.0001 to 0.0010% by mass, Li: 0.0001 to 0.0010% by mass, and REM: 0.0001 to 0.0010% by mass; wherein a total area ratio of bainite, bainitic ferrite, martensite, residual austenite, and a martensite-austenite mixed structure, measured in accordance with the method described in the description, is 95% or more and 100% or less, a total area ratio of ferrite and pearlite, measured in accordance with the method described in the description, is less than 5%, an area ratio of the martensite-austenite mixed structure, measured in accordance with the method described in the description, is 5% or more and 30% or less, an average section length of the martensite-austenite mixed structure, measured in accordance with the method described in the description, is 0.32 μm or less, and a proportion of an area of a region in ferrite, bainitic ferrite, and martensite where no cementite is present to the total area of ferrite, bainitic ferrite, and martensite, measured in accordance with the method described in the description, is 3.0% or more and 5.0% or less.