To provide a power storage unit having a structure which is unlikely to break down by change in shape, such as bending. An electrode plate is covered with a sheet of an insulator which is folded in two. The sheet is preferably processed into a bag-like shape or an envelope-like shape by bonding overlapping portions of the sheet in the periphery of the electrode plate. The electrode plate and the sheet are fixed to an exterior body. In the case where the shape of the exterior body is changed by bending or the like, the electrode plate and the sheet can slide together in the exterior body. Thus, stress on the electrode plate due to bending can be relieved.