A battery pack, comprising: a first battery group in which a plurality of storage batteries having a battery can wide side surface, a battery can narrow side surface, and a battery can bottom surface linked to the battery can wide side surface and the battery can narrow side surface are laminated so that the battery can wide side surfaces face each other; a second battery group in which a plurality of storage batteries having a battery can wide side surface, a battery can narrow side surface, and a battery can bottom surface linked to the battery can wide side surface and the battery can narrow side surface are laminated so that the battery can wide side surfaces face each other; and a case housing the first battery group and the second battery group, wherein the battery can narrow side surfaces of the first battery group and the second battery group are disposed to face each other, a first heat transfer member is disposed between the first battery group and the second battery group, the first heat transfer member is closely adhered on the battery can narrow side surface of the first battery group and the battery can narrow side surface of the second battery group, the first battery group is interposed between the first heat transfer member and a second heat transfer member, the second battery group is interposed between the first heat transfer member and a third heat transfer member, and the battery can wide side surfaces of a storage battery of the first battery group and a storage battery of the second battery group are closely adhered on the bottom surface of the case, characterized in that: a thickness of the first heat transfer member is smaller than thicknesses of the second heat transfer member and the third heat transfer member, and wherein each of the first, second and third heat transfer member is a solid, flat aluminum plate. a first battery group in which a plurality of storage batteries having a battery can wide side surface, a battery can narrow side surface, and a battery can bottom surface linked to the battery can wide side surface and the battery can narrow side surface are laminated so that the battery can wide side surfaces face each other; a second battery group in which a plurality of storage batteries having a battery can wide side surface, a battery can narrow side surface, and a battery can bottom surface linked to the battery can wide side surface and the battery can narrow side surface are laminated so that the battery can wide side surfaces face each other; and a case housing the first battery group and the second battery group, wherein the battery can narrow side surfaces of the first battery group and the second battery group are disposed to face each other, a first heat transfer member is disposed between the first battery group and the second battery group, the first heat transfer member is closely adhered on the battery can narrow side surface of the first battery group and the battery can narrow side surface of the second battery group, the first battery group is interposed between the first heat transfer member and a second heat transfer member, the second battery group is interposed between the first heat transfer member and a third heat transfer member, and the battery can wide side surfaces of a storage battery of the first battery group and a storage battery of the second battery group are closely adhered on the bottom surface of the case, characterized in that: a thickness of the first heat transfer member is smaller than thicknesses of the second heat transfer member and the third heat transfer member, and wherein each of the first, second and third heat transfer member is a solid, flat aluminum plate. a thickness of the first heat transfer member is smaller than thicknesses of the second heat transfer member and the third heat transfer member, and wherein each of the first, second and third heat transfer member is a solid, flat aluminum plate.