A battery capacity calculating method for calculating a residual capacity and/or residual power of a secondary battery, comprising: an equilibrium voltage curve calculating step for calculating an equilibrium voltage curve C equ showing a relation between a discharged capacity and an equilibrium voltage in said secondary battery; a voltage current measuring step for measuring a terminal voltage V mea and a current value I of said secondary battery at a time of discharging; a discharged capacity calculating step for calculating a discharged capacity Q mea of said secondary battery based on the terminal voltage V mea and the current value I, both measured at said voltage current measuring step; an apparent equilibrium voltage calculating step for calculating an apparent equilibrium voltage V ocv by adding a voltage drop ΔV dc by a direct current resistance R dc to the terminal voltage V mea; an apparent discharged capacity calculating step for calculating an apparent discharged capacity Q ocv corresponding to the apparent equilibrium voltage V ocv calculated at said equilibrium voltage calculating step based on the equilibrium voltage curve Q equ calculated at said equilibrium voltage curve calculating step; a capacity shift calculating step for calculating a capacity shift ΔQ being a difference between the discharged capacity Q mea and the apparent discharged capacity Q ocv; and an estimating step for estimating a discharge curve C pre in a future based on the capacity shift ΔQ calculated at said capacity shift calculating step, whereby the discharge curve of said secondary battery shows a relation of the discharged capacities and the terminal voltages of said secondary battery, characterized in that at said estimating step, when a state of said battery is close to the last stage of discharging, a reduction rate dQ of a capacity shift to a discharged capacity is calculated based on the discharged capacity Q mea calculated at said discharged capacity calculating step and the capacity shift ΔQ calculated at said capacity shift calculating step, and the discharge curve C pre in the future is estimated based on the capacity shift ΔQ and the reduction rate dQ. an equilibrium voltage curve calculating step for calculating an equilibrium voltage curve C equ showing a relation between a discharged capacity and an equilibrium voltage in said secondary battery; a voltage current measuring step for measuring a terminal voltage V mea and a current value I of said secondary battery at a time of discharging; a discharged capacity calculating step for calculating a discharged capacity Q mea of said secondary battery based on the terminal voltage V mea and the current value I, both measured at said voltage current measuring step; an apparent equilibrium voltage calculating step for calculating an apparent equilibrium voltage V ocv by adding a voltage drop ΔV dc by a direct current resistance R dc to the terminal voltage V mea; an apparent discharged capacity calculating step for calculating an apparent discharged capacity Q ocv corresponding to the apparent equilibrium voltage V ocv calculated at said equilibrium voltage calculating step based on the equilibrium voltage curve Q equ calculated at said equilibrium voltage curve calculating step; a capacity shift calculating step for calculating a capacity shift ΔQ being a difference between the discharged capacity Q mea and the apparent discharged capacity Q ocv; and an estimating step for estimating a discharge curve C pre in a future based on the capacity shift ΔQ calculated at said capacity shift calculating step, whereby the discharge curve of said secondary battery shows a relation of the discharged capacities and the terminal voltages of said secondary battery,