Disclosed herein is a secondary battery which comprises a film (“safety film”) including expansible graphite that generates inflammable gas at a high temperature and polyurethane that causes an endothermic reaction and generates char at a high temperature. The safety film is a thin member, and therefore, the secondary battery according to the present invention has effects in that the safety film can be easily attached to a region of the secondary battery where the operating components of the secondary battery are not affected. In addition, the graphite component and the polyurethane component included in the safety film provide effective and perfect inflammability by virtue of the inflammable gas and char generated from the graphite component and the polyurethane component, respectively.