A power storage device 20 comprises: a plurality of power storage elements C1-C6 that are connected in series; energy transfer circuits 40 provided respectively to the plurality of power storage elements C1-C6; a common bus 50 to which the energy transfer circuits 40 of the plurality of power storage elements C1-C6 are commonly connected; and a control device 70. Each energy transfer circuit 40 includes one or a plurality of switching transformers Tr, each switching transformer having a first winding 41A that is connected to the power storage elements C1-C6 and a secondary winding 41B that is connected to the common bus 50. The control device 70 uses the switching transformers Tr of the energy transfer circuits 40 to transfer energy between the power storage elements via the common bus 50, thereby equalizing the voltages of the power storage elements C1-C6. The common bus 50 is in an electrically floating state.