In the method for estimating battery residual capacity of the present invention, the voltage measurement values are obtained (step S2). And once it is finished, using the initial values of the coefficients set at the step S3 as starting, the optimization is progressed with renewing each value of the coefficients on the following iterating calculations (step S4). Once the optimum value of each coefficient in the approximation is determined at the step S4, the stable open circuit voltage is calculated by the optimized reciprocal function using thereof at the step S5. And then based thereon, the battery residual capacity is calculated by the predetermined conversion method (step S6).