An electric energy storage device comprising: an electric energy storage element connected to a DC power source via a power converter device; and an output current controller which is connected to the power converter device and which is configured to control charging/discharging current of the electric energy storage element relative to the DC power source, the output current controller being connected with a voltage detector configured to detect a voltage of the DC power source, a charging rate detector configured to detect a charging rate of the electric energy storage element, and a control table having set therein charging/discharging characteristics, and the charging / discharging characteristics are determined based on the voltage of the DC power source detected by the voltage detector and the charging rate of the electric energy storage element detected by the charging rate detector, the voltage of the DC power source is selected from at least one of a charging start voltage, a discharging start voltage, a charging current saturated voltage and a discharging current saturated voltage of the electric energy storage element, the charging/discharging characteristics stored in the control table being set such that at least one of the charging start voltage, the discharging start voltage, the charging current saturated voltage and the discharging current saturated voltage is set to a higher value at higher charging rate of the electric energy storage element than at lower charging rate thereof, and the output current controller is configured to terminate charging or discharging operations within a range from the discharging start voltage to the charging start voltage when charging rate of the electric energy storage element changes. an electric energy storage element connected to a DC power source via a power converter device; and an output current controller which is connected to the power converter device and which is configured to control charging/discharging current of the electric energy storage element relative to the DC power source, the output current controller being connected with a voltage detector configured to detect a voltage of the DC power source, a charging rate detector configured to detect a charging rate of the electric energy storage element, and a control table having set therein charging/discharging characteristics, and the charging / discharging characteristics are determined based on the voltage of the DC power source detected by the voltage detector and the charging rate of the electric energy storage element detected by the charging rate detector, the voltage of the DC power source is selected from at least one of a charging start voltage, a discharging start voltage, a charging current saturated voltage and a discharging current saturated voltage of the electric energy storage element, the charging/discharging characteristics stored in the control table being set such that at least one of the charging start voltage, the discharging start voltage, the charging current saturated voltage and the discharging current saturated voltage is set to a higher value at higher charging rate of the electric energy storage element than at lower charging rate thereof, and the output current controller is configured to terminate charging or discharging operations within a range from the discharging start voltage to the charging start voltage when charging rate of the electric energy storage element changes.