A method of manufacturing a battery pack that is provided with a plurality of stacked unit cells, the method comprising a step of, when a unit cell disposed on one outermost layer is referred to as a first unit cell, and, of the plurality of unit cells stacked in order from the first unit cell, the unit cell disposed on the other outermost layer opposite to that of the first unit cell is referred to as the Nth, where N is a positive number of 2 or more, unit cell: arranging an anvil at an external terminal of the Nth unit cell outside the battery pack; arranging a horizontal-vibration horn either at an external terminal of the (N−1)th unit cell outside the battery pack, or between an external terminal of the (N−1)th unit cell and an external terminal of the (N−2)th unit cell; and then using the anvil and the horn to ultrasonically join the external terminal of the Nth unit cell and the external terminal of the (N−1)th unit cell.