A flexible secondary battery comprising: an electrode assembly that comprises a first electrode layer, a second electrode layer, and a separator that is disposed between the first electrode layer and the second electrode layer; wherein the first electrode layer comprises a first active material unit, the first active material unit being formed as a coating of a first active material on a first metal current collector, and a first non-coated portion on which the first active material is not coated and to which a first electrode tab is attached, and the second electrode layer comprises a second active material unit, the second active material unit being formed as a coating of a second active material on a second metal current collector, and a second non-coated portion on which the second active material is not coated and to which a second electrode tab is attached; a gasket that is flexible and surrounds edges of the electrode assembly, and having first and second ends and first and second sides extending in a longitudinal direction of the electrode assembly; a first sealing sheet that is attached to a first surface of the gasket; and a second sealing sheet that is attached to a second surface of the gasket opposite the first surface; wherein either the first electrode tab and the second electrode tab extend to the outside between the gasket and the first sealing sheet or between the gasket and the second sealing sheet at the first end of the gasket in the longitudinal direction; or the gasket comprises a first lead electrode and a second lead electrode that pass through the gasket at the first end of the gasket in the longitudinal direction, wherein the first lead electrode is adhered to the first electrode tab and the second lead electrode is adhered to the second electrode tab in an internal space of the gasket; wherein an uneven pattern is formed on a bendable area of the gasket at the first and second sides of the gasket wherein at least one dimension selected from a thickness and a width of the gasket varies in the bendable area in the longitudinal direction of the electrode assembly. an electrode assembly that comprises a first electrode layer, a second electrode layer, and a separator that is disposed between the first electrode layer and the second electrode layer; wherein the first electrode layer comprises a first active material unit, the first active material unit being formed as a coating of a first active material on a first metal current collector, and a first non-coated portion on which the first active material is not coated and to which a first electrode tab is attached, and the second electrode layer comprises a second active material unit, the second active material unit being formed as a coating of a second active material on a second metal current collector, and a second non-coated portion on which the second active material is not coated and to which a second electrode tab is attached; a gasket that is flexible and surrounds edges of the electrode assembly, and having first and second ends and first and second sides extending in a longitudinal direction of the electrode assembly; a first sealing sheet that is attached to a first surface of the gasket; and a second sealing sheet that is attached to a second surface of the gasket opposite the first surface; wherein either the first electrode tab and the second electrode tab extend to the outside between the gasket and the first sealing sheet or between the gasket and the second sealing sheet at the first end of the gasket in the longitudinal direction; or the gasket comprises a first lead electrode and a second lead electrode that pass through the gasket at the first end of the gasket in the longitudinal direction, wherein the first lead electrode is adhered to the first electrode tab and the second lead electrode is adhered to the second electrode tab in an internal space of the gasket; wherein an uneven pattern is formed on a bendable area of the gasket at the first and second sides of the gasket wherein at least one dimension selected from a thickness and a width of the gasket varies in the bendable area in the longitudinal direction of the electrode assembly.