A battery includes a power generation element, a case member having an accommodation recess for accommodating the power generation element and including first and second sides, and an opening closure member for closing the accommodation recess. A case inside part of the opening closure member contacts a first center edge of a first edge of the first side to prevent warping deformation of the first edge toward the accommodation recess, and is spaced from a first-first end and a first-second end. In addition, the opening closure member contacts a second center edge of a second edge of the second side to prevent warping deformation of the second edge toward the accommodation recess, and is spaced from a second-first end and a second-second end. The case member and the opening closure member are secured to each other at a welded part around their entire periphery.