A spot-welded joined structure formed by overlapping a plurality of pieces of steel plates (1A, 1B) and performing spot welding on the steel plates (1A, 1B), the spot-welded joined structure comprising: the plurality of pieces of steel plates (1A, 1B), a heat-affected zone, and a nugget, wherein in the plurality of pieces of steel plates (1A, 1B), at least one piece of steel plate is high-strength steel plate whose tensile strength is 750 MPa to 2500 MPa, wherein a carbon equivalent Ceq of the high-strength steel plate represented by the following expression is 0.20 mass% to 0.55 mass%, the spot-welded joined structure being characterized in that in the heat-affected zone, ten or more of iron-based carbides in each of which a length of a longest portion is 0.1 μ m or more exist in a square region whose length of one side is 10 μ m in which a plate thickness direction and a plate surface direction of the steel plates (1A, 1B) are set to a vertical direction and a horizontal direction, respectively, wherein the square region is in a cross section that passes through a center of a welding mark formed on surfaces of the steel plates (1A, 1B) by the spot welding, and is cut along the plate thickness direction of the steel plates (1A, 1B), wherein a position of a center of the square region is a position, at the cross section, separated by 100 μ m from a position of an end portion of the nugget in a direction perpendicular to a tangent to a line indicating the end portion of the nugget, wherein the tangent is a tangent at the position of the end portion of the nugget for determining the square region, and wherein the position of the end portion of the nugget for determining the square region is a position within a range whose center is set to a center in the plate thickness direction of the spot-welded joint and having a length of 1/4 times a total plate thickness being a total value of plate thicknesses of the plurality of pieces of steel plates (1A, 1B) along the plate thickness direction, wherein Ceq = C + Si / 30 + Mn / 20 + 2 P + 4 S, [Si], [Mn],, and in the above expression indicate respective contents (mass%) of C, Si, Mn, P, and S. the plurality of pieces of steel plates (1A, 1B), a heat-affected zone, and a nugget, wherein in the plurality of pieces of steel plates (1A, 1B), at least one piece of steel plate is high-strength steel plate whose tensile strength is 750 MPa to 2500 MPa, wherein a carbon equivalent Ceq of the high-strength steel plate represented by the following expression is 0.20 mass% to 0.55 mass%, the spot-welded joined structure being characterized in that in the heat-affected zone, ten or more of iron-based carbides in each of which a length of a longest portion is 0.1 μ m or more exist in a square region whose length of one side is 10 μ m in which a plate thickness direction and a plate surface direction of the steel plates (1A, 1B) are set to a vertical direction and a horizontal direction, respectively, wherein the square region is in a cross section that passes through a center of a welding mark formed on surfaces of the steel plates (1A, 1B) by the spot welding, and is cut along the plate thickness direction of the steel plates (1A, 1B), wherein a position of a center of the square region is a position, at the cross section, separated by 100 μ m from a position of an end portion of the nugget in a direction perpendicular to a tangent to a line indicating the end portion of the nugget, wherein the tangent is a tangent at the position of the end portion of the nugget for determining the square region, and wherein the position of the end portion of the nugget for determining the square region is a position within a range whose center is set to a center in the plate thickness direction of the spot-welded joint and having a length of 1/4 times a total plate thickness being a total value of plate thicknesses of the plurality of pieces of steel plates (1A, 1B) along the plate thickness direction, wherein Ceq = C + Si / 30 + Mn / 20 + 2 P + 4 S, [Si], [Mn],, and in the above expression indicate respective contents (mass%) of C, Si, Mn, P, and S.