A battery pack has battery cells and bus bars. Each of the battery cells is covered with an exterior case. All the battery cells are electrically connected in series through the bus bars. The battery cells are divided into a first stacked group and a second stacked group arranged adjacent to each other in a lateral direction of the battery pack. The battery cells in each stacked group are stacked in a thickness direction of the battery pack. The stacked groups have a different number of the battery cells in order to make a stair structure. An electrode terminal of one end terminal of the battery cells connected in series is arranged adjacent to the stair structure. A control board is arranged on a lower step part of the stair structure. Devices formed on the control board detect a state of each battery cell.