A flexible secondary battery comprising: an electrode assembly; and external material surrounding the electrode assembly and having at least one bonding portion the external material comprising a first sealing sheet adjacent a first surface of the electrode assembly, a second sealing sheet adjacent a second surface of the electrode assembly opposite to the first surface, and a pair of third sealing sheets that respectively surround a pair of side surfaces of the electrode assembly, wherein the external material is bonded to itself at the at least one bonding portion, the at least one bonding portion comprising a first bonding portion at which an edge region of the first sealing sheet and an edge region of at least one of the pair of third sealing sheets are bonded to each other; wherein the first bonding portion extends from one side of the electrode assembly to an opposite side of the assembly at an edge region of the electrode assembly; and a second bonding portion at which an edge region of the second sealing sheet and an edge region of at least one of the pair of third sealing sheets are bonded to each other, and wherein the second bonding portion extends from one side of the electrode assembly to an opposite side of the electrode assembly along in a lengthwise direction and overlaps the electrode assembly at an edge region of the electrode assembly; wherein each of the first sealing sheet, the second sealing sheet, and the pair of third sealing sheets comprises a first insulating layer, a metal layer, and a second insulating layer that are sequentially stacked, wherein the first insulating layer of the first sealing sheet and the first insulating layer of each of the third sealing sheets are bonded to each other at the first bonding portion, wherein the first insulating layer of the second sealing sheet and the first insulating layer of each of the third sealing sheets are bonded to each other at the second bonding portion, wherein the first insulating layer of the first sealing sheet is on the first surface of the electrode assembly, and the first sealing sheet is folded on itself at the first bonding portion so that a portion of the first insulating layer of the first sealing sheet faces away from the electrode assembly, and wherein the first insulating layer of the second sealing sheet is on the second surface of the electrode assembly, and the second sealing sheet is folded on itself at the second bonding portion so that a portion of the first insulating layer of the second sealing sheet faces away from the electrode assembly; wherein the first bonding portion on the first surface of the electrode assembly is opposite to the second bonding portion on the second surface of the electrode assembly. an electrode assembly; and external material surrounding the electrode assembly and having at least one bonding portion the external material comprising a first sealing sheet adjacent a first surface of the electrode assembly, a second sealing sheet adjacent a second surface of the electrode assembly opposite to the first surface, and a pair of third sealing sheets that respectively surround a pair of side surfaces of the electrode assembly, wherein the external material is bonded to itself at the at least one bonding portion, the at least one bonding portion comprising a first bonding portion at which an edge region of the first sealing sheet and an edge region of at least one of the pair of third sealing sheets are bonded to each other; wherein the first bonding portion extends from one side of the electrode assembly to an opposite side of the assembly at an edge region of the electrode assembly; and a second bonding portion at which an edge region of the second sealing sheet and an edge region of at least one of the pair of third sealing sheets are bonded to each other, and wherein the second bonding portion extends from one side of the electrode assembly to an opposite side of the electrode assembly along in a lengthwise direction and overlaps the electrode assembly at an edge region of the electrode assembly; wherein each of the first sealing sheet, the second sealing sheet, and the pair of third sealing sheets comprises a first insulating layer, a metal layer, and a second insulating layer that are sequentially stacked, wherein the first insulating layer of the first sealing sheet and the first insulating layer of each of the third sealing sheets are bonded to each other at the first bonding portion, wherein the first insulating layer of the second sealing sheet and the first insulating layer of each of the third sealing sheets are bonded to each other at the second bonding portion, wherein the first insulating layer of the first sealing sheet is on the first surface of the electrode assembly, and the first sealing sheet is folded on itself at the first bonding portion so that a portion of the first insulating layer of the first sealing sheet faces away from the electrode assembly, and wherein the first insulating layer of the second sealing sheet is on the second surface of the electrode assembly, and the second sealing sheet is folded on itself at the second bonding portion so that a portion of the first insulating layer of the second sealing sheet faces away from the electrode assembly; wherein the first bonding portion on the first surface of the electrode assembly is opposite to the second bonding portion on the second surface of the electrode assembly.