NOVELTY - The device (1) has multiple battery cells (2) which are layered in layer direction and is electrically connected in series and/or in parallel. A battery casing (4) is provided for accommodating multiple battery cells. The battery casing is composed of heat insulating structure. A heating unit (7) is provided between lower surfaces (2b) of battery cells and bottom plate (42) of the battery casing for heating battery cells. A heat storage layer (8) is provided between heating unit and bottom plate of the battery casing for storing heat generated at heating unit.    USE - Electric power source device for supplying electric power to electric motor of vehicle such as hybrid vehicle. Can also be used in electric vehicle.    ADVANTAGE - The heat is transferred from a lower portion toward an upper portion and temperature difference between the lower and upper portions of the battery casing and the battery cells can be balanced so that the battery cells are effectively heated. Since the heat storage layer for storing the heat generated at the heating unit is provided, heat quantity to be transferred to the lower surfaces of the battery cells can be increased so that the effect for heating the battery cells can be further increased. Multiple louvers can be provided in the fin portion by cutting and bending to further improve the cooling performance. The heat from the heating unit and the heat storage layer is blocked in the cell accommodating space so that warmth retaining property can be improved to properly bring out the battery performance. The heat of the heating unit can be transferred directly or indirectly via the electric insulating layer to the battery cells so that efficiency of the heat transfer can be increased to effectively heat the battery cells. The corrosion of the conducting material for the outer packaging elements of the battery cells can be suppressed. The structure for cooling down the vicinity of the terminals can be simplified so that a number of parts and components as well as manufacturing cost can be lowered. Since the heat collected from the electrical components can be used for heating the battery cells, heating efficiency can be further increased. Since the energy to be supplied to the heating unit can be reduced so that the energy saving can be realized. The cooling air widely and totally flows in the cell accommodating space so that the battery cells can be equally and effectively cooled down. The pressure loss from the air inlet to the outlet opening is uniformized so that the cooling effect is increased.    DESCRIPTION OF DRAWING(S) - The drawing shows schematic cross sectional view showing an inside structure of a battery casing of an electric power source device.    Electric power source device (1)    Multiple battery cells (2)    Lower surfaces (2b)    Battery casing (4)    Heating unit (7)    Heat storage layer (8)    Bottom plate (42)