A secondary battery comprising: a battery electrode assembly that has a quadrangular planar shape, and includes a positive electrode and a negative electrode laminated with a separator interposed therebetween; and an exterior container that is formed of flexible films, and contains said battery electrode assembly, wherein said exterior container has a recessed emboss portion that contains said battery electrode assembly therein, four corners of an outer peripheral portion of said battery electrode assembly are either folded to be contained inside said emboss portion, or inserted between said flexible films outside said emboss portion, a length of at least one side among four sides (4b, 4c) forming the quadrangular planar shape of said battery electrode assembly is shorter than an inner dimension of said emboss portion in a direction in which the one side extends, wherein said four corners of said battery electrode assembly have such size as to protrude to outside of said emboss portion in plan view in a state before being contained in said emboss portion, the corners of the emboss portion are curved, the corners of the battery electrode assembly have right angles, at least respective active material layers of said positive electrode and said negative electrode each have such size so as not to protrude to outside of said emboss portion in plan view., lengths of two sides (4b or 4c) opposite to each other among the four sides forming the quadrangular planar shape of said battery electrode assembly each are shorter than an inner dimension of said emboss portion in a direction in which the two sides (4b or 4c) extend, and lengths of other two sides (4c or 4b) each are shorter than an inner dimension of said emboss portion in a direction in which the other two sides (4c or 4b) extend, or lengths of other two sides (4c or 4b) each are longer than an inner dimension of said emboss portion in a direction in which the other two sides (4c or 4b) extend. a battery electrode assembly that has a quadrangular planar shape, and includes a positive electrode and a negative electrode laminated with a separator interposed therebetween; and an exterior container that is formed of flexible films, and contains said battery electrode assembly, wherein said exterior container has a recessed emboss portion that contains said battery electrode assembly therein, four corners of an outer peripheral portion of said battery electrode assembly are either folded to be contained inside said emboss portion, or inserted between said flexible films outside said emboss portion, a length of at least one side among four sides (4b, 4c) forming the quadrangular planar shape of said battery electrode assembly is shorter than an inner dimension of said emboss portion in a direction in which the one side extends, wherein said four corners of said battery electrode assembly have such size as to protrude to outside of said emboss portion in plan view in a state before being contained in said emboss portion, the corners of the emboss portion are curved, the corners of the battery electrode assembly have right angles, at least respective active material layers of said positive electrode and said negative electrode each have such size so as not to protrude to outside of said emboss portion in plan view., lengths of two sides (4b or 4c) opposite to each other among the four sides forming the quadrangular planar shape of said battery electrode assembly each are shorter than an inner dimension of said emboss portion in a direction in which the two sides (4b or 4c) extend, and lengths of other two sides (4c or 4b) each are shorter than an inner dimension of said emboss portion in a direction in which the other two sides (4c or 4b) extend, or lengths of other two sides (4c or 4b) each are longer than an inner dimension of said emboss portion in a direction in which the other two sides (4c or 4b) extend.