A battery pack, comprising: a plurality of unit batteries, each including a pair of first side surfaces and a pair of second side surfaces that are perpendicular to the pair of first side surfaces, the unit batteries being side-by-side in a first direction such that respective first side surfaces of adjacent said unit batteries face each other; a protective circuit module electrically connected to the plurality of unit batteries, the protective circuit module including a substrate and protective devices mounted on the substrate; a first cap cover accommodating the protective circuit module and the top portions of the plurality of unit batteries; and a second cap cover spaced apart from the first cap cover such that the plurality of unit batteries are therebetween, the second cap cover accommodating bottom portions of the plurality of unit batteries, wherein at least one of the first cap cover and the second cap cover includes a rib extending in a region between an adjacent two of the said unit batteries, characterized in that the first cap cover includes a top plate corresponding to top surfaces of the plurality of unit batteries and first and second skirt portions that are perpendicular to the top plate and are parallel to each other; the first cap cover includes a first rib supporting one side surface of the protective circuit module and extending in the region between the adjacent unit batteries; a first side portion of the first rib is integral with the second skirt portion and a second side portion of the first rib is spaced apart from an inner side surface of the first skirt portion; the protective circuit module is located between the second side portion of the first rib and the inner side surface of the first skirt portion; and in that one surface of the substrate of the protection circuit module, on which the protective devices are mounted, faces the inner side surface of the first skirt portion or the second skirt portion. a plurality of unit batteries, each including a pair of first side surfaces and a pair of second side surfaces that are perpendicular to the pair of first side surfaces, the unit batteries being side-by-side in a first direction such that respective first side surfaces of adjacent said unit batteries face each other; a protective circuit module electrically connected to the plurality of unit batteries, the protective circuit module including a substrate and protective devices mounted on the substrate; a first cap cover accommodating the protective circuit module and the top portions of the plurality of unit batteries; and a second cap cover spaced apart from the first cap cover such that the plurality of unit batteries are therebetween, the second cap cover accommodating bottom portions of the plurality of unit batteries, wherein at least one of the first cap cover and the second cap cover includes a rib extending in a region between an adjacent two of the said unit batteries, characterized in that the first cap cover includes a top plate corresponding to top surfaces of the plurality of unit batteries and first and second skirt portions that are perpendicular to the top plate and are parallel to each other; the first cap cover includes a first rib supporting one side surface of the protective circuit module and extending in the region between the adjacent unit batteries; a first side portion of the first rib is integral with the second skirt portion and a second side portion of the first rib is spaced apart from an inner side surface of the first skirt portion; the protective circuit module is located between the second side portion of the first rib and the inner side surface of the first skirt portion; and in that one surface of the substrate of the protection circuit module, on which the protective devices are mounted, faces the inner side surface of the first skirt portion or the second skirt portion.