A battery system including battery blocks (3) having a plurality of battery cells (1) stacked with cooling gaps (4) established between the battery cells to pass cooling gas; ventilating ducts (5), which are supply ducts (6) and exhaust ducts (7), disposed on both sides of the battery blocks to forcibly ventilate the cooling gaps; and ventilating apparatus (9) to force cooling gas to flow through the ventilating ducts. Cooling gas forcibly introduced by the ventilating apparatus flows from the supply ducts through the cooling gaps and into the exhaust ducts to cool the battery cells. In addition, the battery system has temperature equalizing walls (8) disposed in the supply ducts. The temperature equalizing walls are long and narrow with length in the direction of flow greater than the width, and each temperature equalizing wall gradually narrows towards the upstream end.