A steel sheet consisting of: as chemical components, by mass%, 0.05% to 0.35% of C; 0.05% to 2.0% of Si; 0.8% to 3.0% of Mn; 0.01% to 2.0% of Al; equal to or less than 0.1% of P; equal to or less than 0.05% of S; equal to or less than 0.01% of N; and optionally at least one of: 0.01% to 0.5% of Mo; 0.005% to 0.1% of Nb; 0.005% to 0.2% of Ti; 0.005% to 0.5% of V; 0.05% to 5.0% of Cr; 0.05% to 5.0% of W; 0.0005% to 0.05% of Ca; 0.0005% to 0.05% of Mg; 0.0005% to 0.05% of Zr; 0.0005% to 0.05% of REM; 0.02% to 2.0% of Cu; 0.02% to 1.0% of Ni; and 0.0003% to 0.007% of B; and the balance including iron and inevitable impurities, wherein an area ratio of equal to or higher than 50% of a total of a ferrite phase, a bainite phase, and a tempered martensite phase is contained, an area ratio of equal to or higher than 3% of a retained austenite phase is contained, and a number ratio of equal to or higher than 50% of the crystal grains of the retained austenite phase satisfy Expression 1, assuming that a carbon concentration at a position of center of gravity is Cgc and a carbon concentration at a grain boundary is Cgb: Cgb / Cgc ≥ 1.2 wherein an average grain size of the crystal grains is equal to or less than 10 μm, and an average carbon concentration in the retained austenite phase is equal to or higher than 0.7% and equal to or less than 1.5%, wherein a number ratio of equal to or higher than 40% of the crystal grains are small-diameter crystal grains having an average grain size of equal to or greater than 1 μm and equal to or less than 2 μm, and a number ratio of equal to or higher than 20% of the crystal grains are large-diameter crystal grains having an average grain size of equal to or greater than 2 μm, and wherein a number ratio of equal to or higher than 50% of the small-diameter crystal grains satisfy Expression 2, assuming that a carbon concentration at a position of center of gravity is CgcS and a carbon concentration at a grain boundary is CgbS, and a number ratio of equal to or higher than 50% of the large-diameter crystal grains satisfy Expression 3, assuming that a carbon concentration at a position of center of gravity is CgcL and a carbon concentration at a grain boundary is CgbL: CgbS / CgcS > 1.3 1.3 > CgbL / CgcL > 1.1 as chemical components, by mass%, 0.05% to 0.35% of C; 0.05% to 2.0% of Si; 0.8% to 3.0% of Mn; 0.01% to 2.0% of Al; equal to or less than 0.1% of P; equal to or less than 0.05% of S; equal to or less than 0.01% of N; and 0.05% to 0.35% of C; 0.05% to 2.0% of Si; 0.8% to 3.0% of Mn; 0.01% to 2.0% of Al; equal to or less than 0.1% of P; equal to or less than 0.05% of S; equal to or less than 0.01% of N; and optionally at least one of: 0.01% to 0.5% of Mo; 0.005% to 0.1% of Nb; 0.005% to 0.2% of Ti; 0.005% to 0.5% of V; 0.05% to 5.0% of Cr; 0.05% to 5.0% of W; 0.0005% to 0.05% of Ca; 0.0005% to 0.05% of Mg; 0.0005% to 0.05% of Zr; 0.0005% to 0.05% of REM; 0.02% to 2.0% of Cu; 0.02% to 1.0% of Ni; and 0.0003% to 0.007% of B; and 0.01% to 0.5% of Mo; 0.005% to 0.1% of Nb; 0.005% to 0.2% of Ti; 0.005% to 0.5% of V; 0.05% to 5.0% of Cr; 0.05% to 5.0% of W; 0.0005% to 0.05% of Ca; 0.0005% to 0.05% of Mg; 0.0005% to 0.05% of Zr; 0.0005% to 0.05% of REM; 0.02% to 2.0% of Cu; 0.02% to 1.0% of Ni; and 0.0003% to 0.007% of B; and the balance including iron and inevitable impurities, wherein an area ratio of equal to or higher than 50% of a total of a ferrite phase, a bainite phase, and a tempered martensite phase is contained, an area ratio of equal to or higher than 3% of a retained austenite phase is contained, and a number ratio of equal to or higher than 50% of the crystal grains of the retained austenite phase satisfy Expression 1, assuming that a carbon concentration at a position of center of gravity is Cgc and a carbon concentration at a grain boundary is Cgb: Cgb / Cgc ≥ 1.2 wherein an average grain size of the crystal grains is equal to or less than 10 μm, and an average carbon concentration in the retained austenite phase is equal to or higher than 0.7% and equal to or less than 1.5%, wherein a number ratio of equal to or higher than 40% of the crystal grains are small-diameter crystal grains having an average grain size of equal to or greater than 1 μm and equal to or less than 2 μm, and a number ratio of equal to or higher than 20% of the crystal grains are large-diameter crystal grains having an average grain size of equal to or greater than 2 μm, and wherein a number ratio of equal to or higher than 50% of the small-diameter crystal grains satisfy Expression 2, assuming that a carbon concentration at a position of center of gravity is CgcS and a carbon concentration at a grain boundary is CgbS, and a number ratio of equal to or higher than 50% of the large-diameter crystal grains satisfy Expression 3, assuming that a carbon concentration at a position of center of gravity is CgcL and a carbon concentration at a grain boundary is CgbL: CgbS / CgcS > 1.3 1.3 > CgbL / CgcL > 1.1