A power-generating element containing a stacked electrode assembly (10) and an electrolyte solution is disposed in an accommodating space of a plastic battery case (13). The entirety of the battery case (13) is enclosed air-tightly and liquid-tightly in an outer cover made of an aluminum laminate. An opening (13A) of the battery case (13) is sealed by sealing members (16a, 16b) disposed and melt-bonded so as to cover an opening (13A) of the battery case 13 and to sandwich current collectors (14, 15) of the power-generating element from both sides. The outer cover is sealed at at least a location corresponding to the opening (13A) of the battery case (13).