Disclosed is a flat battery which includes power generating element 18 accommodated in an inner space formed by sealing outer peripheral edges of package members 16 and 17, collector 11a, 13a connected to an electrode plate of power generating element 18 and an electrode tab 14, 15 taken out from the outer peripheral edges of package members 16 and 17. Electrode tab 14, 15 has conducting portion 151 overlapping and joined to collector 11a, 13a and stress relieving portion 152 formed of a material having higher elasticity than that of conducting portion 151. It is thus possible to prevent the occurrence of wrinkles in collector 11a, 13a or electrode tab 14, 15 and separations in weld joints due to a difference in expansion/contraction rate between collector 11a, 13a and electrode tab 14, 15.