A high-strength steel sheet, comprising: a chemical composition represented by, in mass%: C: 0.03% to 0.35%; Si: 0.01% to 2.0%; Mn: 0.3% to 4.0%; Al: 0.01% to 2.0%; P: 0.10% or less; S: 0.05% or less; N: 0.010% or less; Cr: 0.0% to 3.0%; Mo: 0.0% to 1.0%; Ni: 0.0% to 3.0%; Cu: 0.0% to 3.0%; Nb: 0.0% to 0.3%; Ti: 0.0% to 0.3%; V: 0.0% to 0.5%; B: 0.0% to 0.1%; Ca: 0.00% to 0.01%; Mg: 0.00% to 0.01%; Zr: 0.00% to 0.01%; REM: 0.00% to 0.01%; and the balance: Fe and impurities, and a microstructure represented by, in area%, martensite: 5% or more; ferrite: 20% or more; and perlite: 5% or less, wherein an average diameter of martensite grain is 4 μm or less in equivalent circle diameter, a ratio of the number of bulging type martensite grains to the number of martensite grains on grain boundary triple points of a matrix is 70% or more, wherein: the bulging type martensite grain is on one of the grain boundary triple points of the matrix; and at least one of grain boundaries of the bulging type martensite grain, the grain boundaries connecting two adjacent grain boundary triple points of the bulging type martensite grain and grains of the matrix, has a convex curvature to an outer side with respect to line segments connecting the two adjacent grain boundary triple points, and an area ratio represented by VM / A0 is 1.0 or more, wherein: VM denotes a total area of the martensite grains on the grain boundary triple points of the matrix; and A0 denotes a total area of polygons composed of the line segments connecting two adjacent grain boundary triple points of the martensite grains. a chemical composition represented by, in mass%: C: 0.03% to 0.35%; Si: 0.01% to 2.0%; Mn: 0.3% to 4.0%; Al: 0.01% to 2.0%; P: 0.10% or less; S: 0.05% or less; N: 0.010% or less; Cr: 0.0% to 3.0%; Mo: 0.0% to 1.0%; Ni: 0.0% to 3.0%; Cu: 0.0% to 3.0%; Nb: 0.0% to 0.3%; Ti: 0.0% to 0.3%; V: 0.0% to 0.5%; B: 0.0% to 0.1%; Ca: 0.00% to 0.01%; Mg: 0.00% to 0.01%; Zr: 0.00% to 0.01%; REM: 0.00% to 0.01%; and the balance: Fe and impurities, and C: 0.03% to 0.35%; Si: 0.01% to 2.0%; Mn: 0.3% to 4.0%; Al: 0.01% to 2.0%; P: 0.10% or less; S: 0.05% or less; N: 0.010% or less; Cr: 0.0% to 3.0%; Mo: 0.0% to 1.0%; Ni: 0.0% to 3.0%; Cu: 0.0% to 3.0%; Nb: 0.0% to 0.3%; Ti: 0.0% to 0.3%; V: 0.0% to 0.5%; B: 0.0% to 0.1%; Ca: 0.00% to 0.01%; Mg: 0.00% to 0.01%; Zr: 0.00% to 0.01%; REM: 0.00% to 0.01%; and the balance: Fe and impurities, and a microstructure represented by, in area%, martensite: 5% or more; ferrite: 20% or more; and perlite: 5% or less, martensite: 5% or more; ferrite: 20% or more; and perlite: 5% or less, wherein an average diameter of martensite grain is 4 μm or less in equivalent circle diameter, a ratio of the number of bulging type martensite grains to the number of martensite grains on grain boundary triple points of a matrix is 70% or more, wherein: the bulging type martensite grain is on one of the grain boundary triple points of the matrix; and at least one of grain boundaries of the bulging type martensite grain, the grain boundaries connecting two adjacent grain boundary triple points of the bulging type martensite grain and grains of the matrix, has a convex curvature to an outer side with respect to line segments connecting the two adjacent grain boundary triple points, and the bulging type martensite grain is on one of the grain boundary triple points of the matrix; and at least one of grain boundaries of the bulging type martensite grain, the grain boundaries connecting two adjacent grain boundary triple points of the bulging type martensite grain and grains of the matrix, has a convex curvature to an outer side with respect to line segments connecting the two adjacent grain boundary triple points, and an area ratio represented by VM / A0 is 1.0 or more, wherein: VM denotes a total area of the martensite grains on the grain boundary triple points of the matrix; and A0 denotes a total area of polygons composed of the line segments connecting two adjacent grain boundary triple points of the martensite grains. VM denotes a total area of the martensite grains on the grain boundary triple points of the matrix; and A0 denotes a total area of polygons composed of the line segments connecting two adjacent grain boundary triple points of the martensite grains.