An apparatus for estimating a state of a secondary battery comprising a blended positive electrode material, wherein the secondary battery comprises a positive electrode including a first positive electrode material and a second positive electrode material having different operating voltage ranges from each other, a negative electrode including a negative electrode material and a separator interposed therebetween, the apparatus comprising: a sensor unit configured to measure, at time intervals, voltage and current of the secondary battery; and a control unit electrically connected with the sensor unit, characterized in that: the control unit is configured to estimate the state of the secondary battery, wherein the state of the secondary battery comprises a state of charge of at least one of the first positive electrode material and the second positive electrode material, by implementing an Extended Kalman Filter algorithm using a state equation and an output equation, wherein the state equation comprises, as a state parameter, the state of charge of at least one of the first positive electrode material and the second positive electrode material, and the output equation comprises, as an output parameter, the voltage of the secondary battery, wherein the state equation and the output equation are derived from a circuit model comprising a first positive electrode material circuit unit and a second positive electrode material circuit unit respectively corresponding to the first and the second positive electrode materials and being connected in parallel with each other, and optionally, a negative electrode material circuit unit corresponding to the negative electrode material and being connected in series with the first and the second positive electrode material circuit units, and each of the first positive electrode material circuit unit, the second positive electrode material circuit unit, and the negative electrode material circuit unit comprises, as an optional element, an open-circuit voltage element (221a, 222a, 210a) varying voltage according to the state of charge of a corresponding electrode material, and/or an impedance element (221b, 222b, 210b). a sensor unit configured to measure, at time intervals, voltage and current of the secondary battery; and a control unit electrically connected with the sensor unit, characterized in that: the control unit is configured to estimate the state of the secondary battery, wherein the state of the secondary battery comprises a state of charge of at least one of the first positive electrode material and the second positive electrode material, by implementing an Extended Kalman Filter algorithm using a state equation and an output equation, wherein the state equation comprises, as a state parameter, the state of charge of at least one of the first positive electrode material and the second positive electrode material, and the output equation comprises, as an output parameter, the voltage of the secondary battery, wherein the state equation and the output equation are derived from a circuit model comprising a first positive electrode material circuit unit and a second positive electrode material circuit unit respectively corresponding to the first and the second positive electrode materials and being connected in parallel with each other, and optionally, a negative electrode material circuit unit corresponding to the negative electrode material and being connected in series with the first and the second positive electrode material circuit units, and each of the first positive electrode material circuit unit, the second positive electrode material circuit unit, and the negative electrode material circuit unit comprises, as an optional element, an open-circuit voltage element (221a, 222a, 210a) varying voltage according to the state of charge of a corresponding electrode material, and/or an impedance element (221b, 222b, 210b). the control unit is configured to estimate the state of the secondary battery, wherein the state of the secondary battery comprises a state of charge of at least one of the first positive electrode material and the second positive electrode material, by implementing an Extended Kalman Filter algorithm using a state equation and an output equation, wherein the state equation comprises, as a state parameter, the state of charge of at least one of the first positive electrode material and the second positive electrode material, and the output equation comprises, as an output parameter, the voltage of the secondary battery, wherein the state equation and the output equation are derived from a circuit model comprising a first positive electrode material circuit unit and a second positive electrode material circuit unit respectively corresponding to the first and the second positive electrode materials and being connected in parallel with each other, and optionally, a negative electrode material circuit unit corresponding to the negative electrode material and being connected in series with the first and the second positive electrode material circuit units, and each of the first positive electrode material circuit unit, the second positive electrode material circuit unit, and the negative electrode material circuit unit comprises, as an optional element, an open-circuit voltage element (221a, 222a, 210a) varying voltage according to the state of charge of a corresponding electrode material, and/or an impedance element (221b, 222b, 210b).