A bipolar battery and a battery assembly unit that reduces a current density change in a battery element is disclosed. The bipolar battery comprises a battery element configured by alternately stacking a bipolar electrode and an electrolyte layer, as well as cathode and anode terminal plates electrically connected to the battery element so as to extract the current from the battery element. In the bipolar battery, the total electrical resistance of the cathode and anode terminal plates along the surface direction is smaller than the total electrical resistance of the battery element along the stacking direction between the cathode and anode terminal plates.