An uninterruptible power supply unit is configured such that, during charging of a first battery pack, and until, after the first battery pack is charged to a fully charged state, the voltage of the first battery pack is lowered below an upper limit voltage of a load device, discharging of the first battery pack is prohibited, and discharging of the second battery pack is allowed, and such that, during charging of a second battery pack, and until, after the second battery pack is charged to a fully charged state, the voltage of the second battery pack is lowered below the upper limit voltage of the load device, discharging of the second battery pack is prohibited, and discharging of the first battery pack is allowed.