A battery pack comprising: a battery element 2 having a cathode, an anode and polymer electrolyte, terminals being drawn from the cathode and the anode, respectively; a first package body 6 having a housing recessed part 61 for housing the battery element and a uniting piece 62 provided in the periphery of the housing recessed part; a second package body 7, harder than the first package body 6, having at least a first area 71 with which a first surface of the battery element housed in the housing recessed part 54 of the first package body which is exposed outside is covered and a second area 72 provided continuously to one side of the first area; a frame 4 disposed in the periphery of the housing recessed part of the first package body in which the battery element is housed; and a connecting board 3 disposed in the frame to which the terminals are connected, wherein the battery element is housed in the housing recessed part of the first package body, the first surface of the battery element exposed outside from the housing recessed part is covered with the first area of the second package body, the uniting piece of the first package body is united to the first area of the second package body, the terminals of the battery element are drawn outside, the terminals of the battery element housed in the housing recessed part of the first package body and covered with the first area of the second package body are connected to the connecting board, the frame is disposed in the periphery of the housing recessed part in which the battery element is housed, the connecting board is disposed in the frame, the second area of the second package body is bent back to a second surface side opposite to the first surface of the battery element housed in the housing recessed part of the first package body relative to the first area united to the uniting piece of the first package body by using the one side of the first area as a supporting point to cover the housing recessed part of the first package body with the second area of the second package body and unite the second area of the second package body to the frame. a battery element 2 having a cathode, an anode and polymer electrolyte, terminals being drawn from the cathode and the anode, respectively; a first package body 6 having a housing recessed part 61 for housing the battery element and a uniting piece 62 provided in the periphery of the housing recessed part; a second package body 7, harder than the first package body 6, having at least a first area 71 with which a first surface of the battery element housed in the housing recessed part 54 of the first package body which is exposed outside is covered and a second area 72 provided continuously to one side of the first area; a frame 4 disposed in the periphery of the housing recessed part of the first package body in which the battery element is housed; and a connecting board 3 disposed in the frame to which the terminals are connected, wherein the battery element is housed in the housing recessed part of the first package body, the first surface of the battery element exposed outside from the housing recessed part is covered with the first area of the second package body, the uniting piece of the first package body is united to the first area of the second package body, the terminals of the battery element are drawn outside, the terminals of the battery element housed in the housing recessed part of the first package body and covered with the first area of the second package body are connected to the connecting board, the frame is disposed in the periphery of the housing recessed part in which the battery element is housed, the connecting board is disposed in the frame, the second area of the second package body is bent back to a second surface side opposite to the first surface of the battery element housed in the housing recessed part of the first package body relative to the first area united to the uniting piece of the first package body by using the one side of the first area as a supporting point to cover the housing recessed part of the first package body with the second area of the second package body and unite the second area of the second package body to the frame.