Flexible battery and method for producing the same

A method for producing a flexible battery comprising a sheet-like electrode group, an electrolyte, and a housing with flexibility enclosing the electrode group and the electrolyte, the method comprising the steps of:

• forming a film material into a corrugated shape having a plurality of ridge lines and a plurality of valley lines arranged in parallel to each other, and then folding the film material into two along a fold line parallel to the ridge lines and the valley lines, such that the ridge lines in one of portions to be opposed to each other overlap the valley lines in the other of the portions, or alternatively, folding a film material into two along the fold line, and then forming the film material into a corrugated shape having a plurality of ridge lines and a plurality of valley lines arranged in parallel to each other, such that the ridge lines in one of portions to be opposed to each other overlap the valley lines in the other of the portions;
• inserting the electrode group into the film material folded into two along the fold line;
• and
• bonding two bonding margins to each other, the bonding margins being set on sides around two facing portions respectively facing two principal surfaces of the electrode group, except a side including the fold line.