A thin battery (10; 20) comprising: an electrode assembly in sheet form comprising at least one electrode structure, the electrode structure being a laminate including: a positive electrode; two negative electrodes arranged so as to sandwich the positive electrode; and two electrolyte layers, one interposed between the positive electrode and one of the negative electrodes, and the other interposed between the positive electrode and the other of the negative electrodes; and a film-made housing for hermetically accommodating the electrode assembly, the positive electrode including a positive electrode current collector, and two positive electrode active material layers formed on both surfaces, respectively, of the positive electrode current collector, the negative electrodes each including a negative electrode current collector, and a negative electrode active material layer arranged on one surface of the negative electrode current collector so as to be in contact with the electrolyte layer, the negative electrodes being electrically connected to each other, the electrode assembly having a bending elastic modulus of 300 MPa or less as measured in compliance with the measurement method of JIS K7171, characterized by the film-made housing being formed from a laminate film (14; 21, 22) comprising a first resin film, and a gas barrier layer and a second resin film laminated in recited order on one surface of the first resin film, the gas barrier layer including a metal material, the gas barrier layer having an average thickness of 30 μm or less, and the electrode assembly and the film-made housing, in total, having a thickness of 1 mm or less. an electrode assembly in sheet form comprising at least one electrode structure, the electrode structure being a laminate including: a positive electrode; two negative electrodes arranged so as to sandwich the positive electrode; and two electrolyte layers, one interposed between the positive electrode and one of the negative electrodes, and the other interposed between the positive electrode and the other of the negative electrodes; and a film-made housing for hermetically accommodating the electrode assembly, the positive electrode including a positive electrode current collector, and two positive electrode active material layers formed on both surfaces, respectively, of the positive electrode current collector, the negative electrodes each including a negative electrode current collector, and a negative electrode active material layer arranged on one surface of the negative electrode current collector so as to be in contact with the electrolyte layer, the negative electrodes being electrically connected to each other, the electrode assembly having a bending elastic modulus of 300 MPa or less as measured in compliance with the measurement method of JIS K7171, characterized by the film-made housing being formed from a laminate film (14; 21, 22) comprising a first resin film, and a gas barrier layer and a second resin film laminated in recited order on one surface of the first resin film, the gas barrier layer including a metal material, the gas barrier layer having an average thickness of 30 μm or less, and the electrode assembly and the film-made housing, in total, having a thickness of 1 mm or less.