A secondary battery capacity measurement system comprising: a data convertor configured to determine a partial derivative characteristic curve of a capacity-to-voltage derivative dQ/dV over voltage, the partial derivative characteristic curve indicating a correspondence between the derivative value dQ/dV and a voltage value V, from a set of historical data of actually time-sequentially-measured values of voltage and current; an SOC computer configured to compute a difference between the partial derivative characteristic curve and a reference derivative curve indicating a reference characteristic of the capacity-to-voltage derivative, and configured to fit the reference derivative curve to the partial derivative characteristic curve by reducing the difference to estimate an SOC; and a maximum capacity computer configured to estimate a maximum value of capacity by integrating the derivative value dQ/dV by the voltage value V over the voltage range of use, wherein the reference derivative curve is given by a complex of first and second characteristic derivative curves, the first characteristic derivative curve is derived from a positive material of a battery positive electrode, and the second characteristic derivative curve is derived from a negative material of a battery negative electrode, wherein the reference derivative curve is given by a separating curve complex of the first and second characteristic derivative curves, and wherein the first and second characteristic derivative curves are approximated with first and second functions which include first and second characteristic parameters of phase transition of the positive material and the negative material, respectively. a data convertor configured to determine a partial derivative characteristic curve of a capacity-to-voltage derivative dQ/dV over voltage, the partial derivative characteristic curve indicating a correspondence between the derivative value dQ/dV and a voltage value V, from a set of historical data of actually time-sequentially-measured values of voltage and current; an SOC computer configured to compute a difference between the partial derivative characteristic curve and a reference derivative curve indicating a reference characteristic of the capacity-to-voltage derivative, and configured to fit the reference derivative curve to the partial derivative characteristic curve by reducing the difference to estimate an SOC; and a maximum capacity computer configured to estimate a maximum value of capacity by integrating the derivative value dQ/dV by the voltage value V over the voltage range of use, wherein the reference derivative curve is given by a complex of first and second characteristic derivative curves, the first characteristic derivative curve is derived from a positive material of a battery positive electrode, and the second characteristic derivative curve is derived from a negative material of a battery negative electrode, wherein the reference derivative curve is given by a separating curve complex of the first and second characteristic derivative curves, and wherein the first and second characteristic derivative curves are approximated with first and second functions which include first and second characteristic parameters of phase transition of the positive material and the negative material, respectively.