An electricity accumulator which is capable of equalizing the voltages of a plurality of electricity accumulating devices such as batteries in an inexpensive and simple construction and quickly even in a state which is not a fully charged state. Dissipation of electrical energy is prevented. The electricity device includes a plurality connected in series. There is a capacitor connectable in parallel with at least two of the electricity accumulating devices. A connection switching device switches selectively a first connection mode in which the capacitor is connected to a first electricity accumulating device so that the capacitor is charged or discharged through the first electricity accumulating device and a second connection mode in which the capacitor is connected to a second electricity accumulating device.