A lithium secondary battery of the present invention comprises: a positive electrode comprising a positive electrode current collector and a positive electrode material mixture layer, a negative electrode comprising a negative electrode current collector and a lithium metal foil, and a separator, and further comprises a thermally deformable separator disposed in the vicinity of the positive electrode material mixture layer on the surface of the positive electrode current collector, and an extension portion that extends from the lithium metal foil so as to be in contact with the thermally deformable separator; the thermally deformable separator having a melting point of 80° C. or higher, which is equal to or lower than the melting point of lithium metal; and when a thermally deformable separator 31 thermally deforms, the extension portion comes into contact with the positive electrode current collector, resulting in a short circuit of the positive electrode and the negative electrode.