A negative lower gasket (17) has a rib (17h) disposed along the outer periphery of a flat portion (17b) having a base of a negative current collector (12) disposed thereat. The height of the rib (H) is set to be more than a thickness of the negative lower gasket (17). The rib may be disposed only at a portion corresponding to a position where legs project from the base of the negative current collector (12) at the outer periphery of the flat portion (17b).