A lid member of a battery case has a protruding section including an inside-outside direction an outside tapered surface, and a top surface between the inside tapered surface and the outside tapered surface. In a cross section, the area of a first imaginary area enclosed by a first line segment of the inside tapered surface, a first imaginary line extending downward from the top of the first line segment, and a second imaginary line extending in an inside-outside direction from the top surface, is smaller than the area of a second imaginary area enclosed by a second line segment of the outside tapered surface, a third imaginary line extending downward from the top of the second line segment, and the second imaginary line.