An apparatus for estimating state of a hybrid secondary battery, wherein the hybrid secondary battery comprises a first secondary battery and a second secondary battery having different electrochemical characateristics from each other with at least one difference in a positive electrode material, a negative electrode material or an electrolyte and being connected in parallel with each other, the apparatus comprising: a sensor unit configured to measure, at time intervals, an operating voltage and an operating current of the hybrid secondary battery; and a control unit electrically connected with the sensor unit, and configured to estimate the state of the hybrid secondary battery, wherein the state comprises at least one of a first state of charge of the first secondary battery and a second state of charge of the second secondary battery, by implementing an Extended Kalman Filter algorithm using a state equation and an output equation, wherein the state equation comprises, as a state parameter, at least one of the first state of charge of the first secondary battery and the second state of charge of the second secondary battery, and the output equation comprises, as an output parameter, the operating voltage of the hybrid secondary battery, wherein the control unit is configured to use the state equation and the output equation derived from a first circuit unit and a second circuit unit which correspond to the first secondary battery and the second secondary battery, respectively, and connected in parallel with each other, and at least one of the first circuit unit and the second circuit unit comprises an open-circuit voltage element (210a, 220a) configured to vary voltage according to a state of charge of the corresponding first or second secondary battery, and optionally, an impedance element (210b, 220b) configured to vary voltage according to current flowing through the corresponding circuit unit. a sensor unit configured to measure, at time intervals, an operating voltage and an operating current of the hybrid secondary battery; and a control unit electrically connected with the sensor unit, and configured to estimate the state of the hybrid secondary battery, wherein the state comprises at least one of a first state of charge of the first secondary battery and a second state of charge of the second secondary battery, by implementing an Extended Kalman Filter algorithm using a state equation and an output equation, wherein the state equation comprises, as a state parameter, at least one of the first state of charge of the first secondary battery and the second state of charge of the second secondary battery, and the output equation comprises, as an output parameter, the operating voltage of the hybrid secondary battery, wherein the control unit is configured to use the state equation and the output equation derived from a first circuit unit and a second circuit unit which correspond to the first secondary battery and the second secondary battery, respectively, and connected in parallel with each other, and at least one of the first circuit unit and the second circuit unit comprises an open-circuit voltage element (210a, 220a) configured to vary voltage according to a state of charge of the corresponding first or second secondary battery, and optionally, an impedance element (210b, 220b) configured to vary voltage according to current flowing through the corresponding circuit unit.