To enable a battery pack to easily release heat generated by unit batteries into an atmosphere, while securing electric insulation among unit battery cases.

The battery pack integrates a plurality of unit batteries 1 put in a row each having a rectangular parallelepiped enclosed metal unit battery case 2. The unit battery cases 2 are arranged with their adjacent metal-case outer surfaces arrayed parallel at intervals each outside surface in a bare state, and a separator 15 of an electric insulating material is arranged between each two adjacent case outer surfaces to form a cooling flow channel. The separator 15 may be provided with a flat part 21 extended along the adjacent case outer surfaces and a plurality of protruded parts protruded toward the adjacent case outer surfaces.