A high-strength steel sheet having a tensile strength of not less than 980 MPa and having a chemical composition comprising, in mass%: C: 0.10% to 0.35%, Si: 0.5% to 2.0%, Mn: 1.5% to 3.0%, P: not more than 0.050%, S: not more than 0.0100%, Al: 0.001% to 1.00%, N: 0.0005% to 0.0200%, optionally one, or two or more selected from Ti: 0.005% to 0.100%, Nb: 0.005% to 0.100%, and V: 0.005% to 0.100%, optionally one, or two or more selected from Cr: 0.05% to 1.0%, Ni: 0.05% to 0.50%, Mo: 0.05% to 1.0%, Cu: 0.005% to 0.500%, and B: 0.0001% to 0.0100%, and optionally either or both of Ca: 0.0001% to 0.0050%, and REM: 0.0005% to 0.0050%, the balance being iron and inevitable impurities, C/Mn being 0.08 to 0.20, the steel sheet having microstructures including, in terms of area fraction relative to all the microstructures: 40% to 70% total of ferrite and bainitic ferrite, 5% to 35% martensite, and 5% to 30% retained austenite, a proportion of martensite which includes retained austenite and is adjacent to bainitic ferrite, i.e. martensite which includes retained austenite and is in contact with bainitic ferrite in at least part of its boundary with adjacent microstructures but is not at all in contact with ferrite on its boundary with adjacent microstructures, being not less than 60% of all martensite including retained austenite, a proportion of differences in microhardness of 4.0 GPa and smaller measured at 0.5 μ m intervals being not less than 70% of all indentations, a proportion of microstructures with a microhardness of 8.0 GPa or smaller being not less than 85% of all the microstructures, wherein the microhardness is measured with a nano indenter by indenting a total of 550 points at 0.5 μ m intervals at 250 μ N load on a face at 1/4 sheet thickness exposed by electrolytic polishing, and the difference in microhardness is obtained by calculating the largest value of the differences in microhardness between one site and each of its adjacent sites, i.e. each of four sites which are left of it, right of it, above it and below it. C: 0.10% to 0.35%, Si: 0.5% to 2.0%, Mn: 1.5% to 3.0%, P: not more than 0.050%, S: not more than 0.0100%, Al: 0.001% to 1.00%, N: 0.0005% to 0.0200%, optionally one, or two or more selected from Ti: 0.005% to 0.100%, Nb: 0.005% to 0.100%, and V: 0.005% to 0.100%, optionally one, or two or more selected from Cr: 0.05% to 1.0%, Ni: 0.05% to 0.50%, Mo: 0.05% to 1.0%, Cu: 0.005% to 0.500%, and B: 0.0001% to 0.0100%, and optionally either or both of Ca: 0.0001% to 0.0050%, and REM: 0.0005% to 0.0050%, the balance being iron and inevitable impurities, C/Mn being 0.08 to 0.20, the steel sheet having microstructures including, in terms of area fraction relative to all the microstructures: 40% to 70% total of ferrite and bainitic ferrite, 5% to 35% martensite, and 5% to 30% retained austenite, a proportion of martensite which includes retained austenite and is adjacent to bainitic ferrite, i.e. martensite which includes retained austenite and is in contact with bainitic ferrite in at least part of its boundary with adjacent microstructures but is not at all in contact with ferrite on its boundary with adjacent microstructures, being not less than 60% of all martensite including retained austenite, a proportion of differences in microhardness of 4.0 GPa and smaller measured at 0.5 μ m intervals being not less than 70% of all indentations, a proportion of microstructures with a microhardness of 8.0 GPa or smaller being not less than 85% of all the microstructures, wherein the microhardness is measured with a nano indenter by indenting a total of 550 points at 0.5 μ m intervals at 250 μ N load on a face at 1/4 sheet thickness exposed by electrolytic polishing, and the difference in microhardness is obtained by calculating the largest value of the differences in microhardness between one site and each of its adjacent sites, i.e. each of four sites which are left of it, right of it, above it and below it.