NOVELTY - The case inside part of an opening closure contacts the first center edge (2313) of first edge (2310) of a first side to prevent warping deformation of the first edge toward an accommodation recess and is spaced from the first-first end (2311) and the first-second end (2312). The case inside part of opening closure further contacts the second center edge (2323) of second edge (2320) of a second side to prevent warping deformation of second edge toward the accommodation recess and is spaced from the second-first end (2321) and second-second end (2322).    USE - Secondary battery e.g. nickel hydrogen battery, lithium ion battery for use as driving source for electric vehicle and hybrid vehicle.    ADVANTAGE - Prevents reduction in sealing strength caused by inward deformation of warping of the first side while welding the first edge of first side and case contact of opening closure to each other. Prevents case inside part and case from rubbing against each other since center edges of the sides deform easily, thus prevents generation of metal powder by inserting the case inside part into the case while deforming the central edges to swell outwardly. Ensures more appropriate welding and provides more stable sealing strength at the first and second center edges since first and second center edges contact the first and second inside part side surfaces entirely in the direction along the first and second longest straight edges, respectively to make changes in welding position at the first and second center edges difficult while welding the first and second sides and case contact part of the opening closure to each other.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a method for manufacturing a battery that includes a power generation element.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross sectional view of the battery.    First edge (2310)    First-first end (2311)    First-second end (2312)    First center edge (2313)    Second edge (2320)    Second-first end (2321)    Second-second end (2322)    Second center edge (2323)