Secondary battery 1 includes: battery electrode assembly 5 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 7, and contains battery electrode assembly 5. The exterior container has recessed emboss portion 8 that contains battery electrode assembly 5 therein. Four corners 4a of an outer peripheral portion of battery electrode assembly 5 are either folded to be contained inside emboss portion 8, or inserted between flexible films 7 outside emboss portion 8. A length of at least one side among four sides forming the quadrangular planar shape of battery electrode assembly 5 is shorter than an inner dimension of emboss portion 8 in the direction in which the one side extends.