This flexible battery is provided with a sheet-form electrode group, an electrolyte, and a flexible outer case housing said electrode group and electrolyte. The outer case contains a film material which has been folded into two parts so as to sandwich the electrode group therebetween. The film material includes two facing portions opposite of the two primary surfaces of the electrode group, a fold between the two facing portions where said film material is folded, and two bonding components positioned around the two facing portions. The two bonding components are bonded to one another to form a bonding unit. At least the two facing portions of one film material are formed to have a wave-form with multiple ridge lines and multiple valley lines parallel to one another. The ridge lines formed on one of the two facing portions and the valley lines formed on the other of said two facing portions overlap. The fold is parallel to the ridge lines and the valley lines.