An outer peripheral surface of a cylindrical cell is coated with a heat insulation material. A heat-resistant material is stacked radially outside the heat insulation material. An electrical insulation material is stacked radially outside the heat-resistant material. The order in which the heat insulation material, the heat-resistant material and the electrical insulation material are stacked one on top of another may be changed. A coating material other than the heat insulation material, the heat-resistant material and the electrical insulation material may be provided.