When an accumulated charge amount of a first slave battery reaches a preset accumulated charge amount or less, the first slave battery is set as a used secondary battery, and when a master battery and a second slave battery are connected to sides of motors, accumulated charge amount differences of the master battery and the second slave battery are respectively set as allowable discharge electric energies E1 and E3, value 0 is set as an allowable discharge electric energy E2, and a sum of these allowable discharge electric energies is set as an EV priority allowable electric energy that is an electric energy that is allowed to be discharged as a whole of the master battery, and the two slave batteries.