A highly formable high-strength steel sheet having a chemical composition consisting of: C in a content of 0.05% to 0.3%; Si in a content of 1% to 3%; Mn in a content of 0.5% to 3%; P in a content of 0% to 0.1%; S in a content of 0% to 0.01%; Al in a content of 0.001% to 0.1%; N in a content of 0.002% to 0.03%; optionally, at least one element selected from the group consisting of: Cr in a content of 0.01% to 3%; Mo in a content of 0.01% to 1%; Cu in a content of 0.01% to 2%; Ni in a content of 0.01% to 2%; and B in a content of 0.00001% to 0.01%; optionally, at least one element selected from the group consisting of: Ca in a content of 0.0005% to 0.01%; Mg in a content of 0.0005% to 0.01%; and a rare-earth element in a content of 0.0001 % to 0.01%, in mass percent, with the balance consisting of iron and inevitable impurities; wherein the steel sheet has a structure consisting of: bainitic ferrite in a content of 50% to 90%; retained austenite in a content of 5% to 20%; martensite and the retained austenite in a total content of 16% to 50%; polygonal ferrite in a content of 0% to 40%; and optionally bainite in a content of 0% to 5%, in area percent based on the entire structure; wherein martensite is partially introduced into the structure; wherein the retained austenite has a carbon content (C γR) of 0.5% to 1.2% in mass percent, an average equivalent circle diameter of 0.2 to 2 μm, and an average aspect ratio of less than 3.0, where an aspect ratio of a retained austenite grain is expressed as a ratio of a maximum diameter to a minimum diameter of the grain; wherein at room temperature, the tensile strength is 980 MPa or more, and the elongation is 15% or more when the tensile strength is less than 1180 MPa, and the elongation is 13% or more when the tensile strength is equal to or more than 1180 MPa; and wherein the steel sheet is cold-rolled, and optionally galvanized or galvannealed. C in a content of 0.05% to 0.3%; Si in a content of 1% to 3%; Mn in a content of 0.5% to 3%; P in a content of 0% to 0.1%; S in a content of 0% to 0.01%; Al in a content of 0.001% to 0.1%; N in a content of 0.002% to 0.03%; Cr in a content of 0.01% to 3%; Mo in a content of 0.01% to 1%; Cu in a content of 0.01% to 2%; Ni in a content of 0.01% to 2%; and B in a content of 0.00001% to 0.01%; Ca in a content of 0.0005% to 0.01%; Mg in a content of 0.0005% to 0.01%; and a rare-earth element in a content of 0.0001 % to 0.01%, in mass percent, with the balance consisting of iron and inevitable impurities; bainitic ferrite in a content of 50% to 90%; retained austenite in a content of 5% to 20%; martensite and the retained austenite in a total content of 16% to 50%; polygonal ferrite in a content of 0% to 40%; and optionally bainite in a content of 0% to 5%, in area percent based on the entire structure; wherein martensite is partially introduced into the structure; wherein the retained austenite has a carbon content (C γR) of 0.5% to 1.2% in mass percent, an average equivalent circle diameter of 0.2 to 2 μm, and an average aspect ratio of less than 3.0, where an aspect ratio of a retained austenite grain is expressed as a ratio of a maximum diameter to a minimum diameter of the grain; wherein at room temperature, the tensile strength is 980 MPa or more, and the elongation is 15% or more when the tensile strength is less than 1180 MPa, and the elongation is 13% or more when the tensile strength is equal to or more than 1180 MPa; and wherein the steel sheet is cold-rolled, and optionally galvanized or galvannealed.