An outer case (10, 20, 35, 40) for housing a rechargeable battery (1, 31, 41) and a battery circuit therein is composed of an upper case (7, 21, 42) and a lower case (8, 22, 43), each being formed as a half-shell body. The respective bottom faces of the upper case (7, 21, 42) and the lower case (8, 22, 43) are formed of film members (7b, 8b, 32, 34, 42b, 43b), and are integrally formed with resin moldings (7a, 8a, 33, 42c, 43c) forming circumferential side faces.