A secondary battery with high capacity per unit volume can be provided. A flexible secondary battery with a novel structure can be provided. A secondary battery that can be bent repeatedly can be provided. A highly reliable secondary battery can be provided. A long-life secondary battery can be provided. A secondary battery comprises an inner structure and an exterior body that surrounds the inner structure. The inner structure comprises a positive electrode and a negative electrode. The exterior body comprises a first exterior film and a second exterior film. A region comprising reduced graphene oxide lies between the first exterior film and the second exterior film. The graphene oxide preferably comprises a region where the concentration of oxygen is higher than or equal to 2 atomic percent and lower than or equal to 20 atomic percent.