An apparatus for estimating voltage of a hybrid secondary battery, wherein the hybrid secondary battery comprises a first secondary battery and a second secondary battery having different electrochemical characteristics from each other and being connected in parallel with each other, the apparatus comprising: a sensor unit configured to measure an operating current of the hybrid secondary battery; and a control unit configured to estimate the voltage of the hybrid secondary battery using the operating current, and a voltage equation derived from a circuit model comprising a first circuit unit configured to simulate a voltage variation of the first secondary battery by a first open-circuit voltage element and a first impedance element, and a second circuit unit connected in parallel with the first circuit unit and configured to simulate a voltage variation of the second secondary battery by a second open-circuit voltage element and a second impedance element, characterized in that the control unit is configured to: determine, from the circuit model, a first current and a second current flowing through the first circuit unit and the second circuit unit, respectively, time-update a first state of charge of the first secondary battery and a second state of charge of the second secondary battery, by adding up the first current and the second current, respectively, time-update a first impedance voltage formed by the first impedance element and a second impedance voltage formed by the second impedance element, by using the time-updated first state of charge and the time-updated second state of charge, and the first current and the second current, determine a first open-circuit voltage of the first secondary battery and a second open-circuit voltage of the second secondary battery corresponding to the time-updated first state of charge and the time-updated second state of charge, and estimate the voltage of the hybrid secondary battery by substituting the determined first open-circuit voltage and the determined second open-circuit voltage, the time-updated first impedance voltage and the time-updated second impedance voltage, and the measured operating current, into the voltage equation. a sensor unit configured to measure an operating current of the hybrid secondary battery; and a control unit configured to estimate the voltage of the hybrid secondary battery using the operating current, and a voltage equation derived from a circuit model comprising a first circuit unit configured to simulate a voltage variation of the first secondary battery by a first open-circuit voltage element and a first impedance element, and a second circuit unit connected in parallel with the first circuit unit and configured to simulate a voltage variation of the second secondary battery by a second open-circuit voltage element and a second impedance element, characterized in that the control unit is configured to: determine, from the circuit model, a first current and a second current flowing through the first circuit unit and the second circuit unit, respectively, time-update a first state of charge of the first secondary battery and a second state of charge of the second secondary battery, by adding up the first current and the second current, respectively, time-update a first impedance voltage formed by the first impedance element and a second impedance voltage formed by the second impedance element, by using the time-updated first state of charge and the time-updated second state of charge, and the first current and the second current, determine a first open-circuit voltage of the first secondary battery and a second open-circuit voltage of the second secondary battery corresponding to the time-updated first state of charge and the time-updated second state of charge, and estimate the voltage of the hybrid secondary battery by substituting the determined first open-circuit voltage and the determined second open-circuit voltage, the time-updated first impedance voltage and the time-updated second impedance voltage, and the measured operating current, into the voltage equation. determine, from the circuit model, a first current and a second current flowing through the first circuit unit and the second circuit unit, respectively, time-update a first state of charge of the first secondary battery and a second state of charge of the second secondary battery, by adding up the first current and the second current, respectively, time-update a first impedance voltage formed by the first impedance element and a second impedance voltage formed by the second impedance element, by using the time-updated first state of charge and the time-updated second state of charge, and the first current and the second current, determine a first open-circuit voltage of the first secondary battery and a second open-circuit voltage of the second secondary battery corresponding to the time-updated first state of charge and the time-updated second state of charge, and estimate the voltage of the hybrid secondary battery by substituting the determined first open-circuit voltage and the determined second open-circuit voltage, the time-updated first impedance voltage and the time-updated second impedance voltage, and the measured operating current, into the voltage equation.