The disclosed secondary battery is a flat secondary battery 10 including an electrode group 1 which is sealed in a laminate case 9 made of laminate films, and includes a positive electrode including a positive electrode current collector carrying thereon a positive electrode material mixture layer containing a positive electrode active material and a binder, a negative electrode, and a porous insulating layer. The laminate case 9 includes a container portion 9a for containing the electrode group 1, a weld portion 9b in which the laminate films are welded to each other, and a non-weld portion 9c which is provided between the container portion 9a and the weld portion 9b, and in which the laminate films are not welded to each other. The positive electrode has a tensile extension of 3.0% or higher.