In a sealed battery pack comprising a plurality of serially disposed cells in which power generating elements are housed in prismatic cell cases and the openings thereof are sealed, the plurality of serially disposed cells are housed in a cooling box, and the top openings in the cooling box and the openings in the various cell cases of the cells are sealed by integrating lids. Coolant passages on both sides between the side surfaces of the cells and the side walls on both sides of the cooling box in the cell arrangement direction, and coolant passages between the cases of the cells are formed so as to communicate with each other, so that all of the side surfaces of the cells are forcibly cooled by a coolant.