A battery capacity estimation apparatus characterized in that the battery capacity estimation apparatus comprises: a current integrated value calculation unit configured to calculate a current integrated value by integrating electric currents of a secondary battery system (23-1,..., 23-M) whose capacity is to be estimated, the electric currents being detected by a current sensor; an SOC estimate-value calculation unit configured to calculate an SOC estimate value, based on a closed circuit voltage, in a stabilization state where a change in SOC of a secondary battery per unit time is comparatively small, the secondary battery constituting the secondary battery system; a regression analysis unit configured to perform a regression analysis in which the current integrated value is defined as a dependent variable and the SOC estimate value is defined as an independent variable, the regression analysis being performed while correcting the current integrated value based on a value of a coefficient of determination so that a result of the regression analysis has predetermined accuracy; and an estimation unit configured to estimate a capacity of the secondary battery system based on the result of the regression analysis. characterized in that the battery capacity estimation apparatus comprises: a current integrated value calculation unit configured to calculate a current integrated value by integrating electric currents of a secondary battery system (23-1,..., 23-M) whose capacity is to be estimated, the electric currents being detected by a current sensor; an SOC estimate-value calculation unit configured to calculate an SOC estimate value, based on a closed circuit voltage, in a stabilization state where a change in SOC of a secondary battery per unit time is comparatively small, the secondary battery constituting the secondary battery system; a regression analysis unit configured to perform a regression analysis in which the current integrated value is defined as a dependent variable and the SOC estimate value is defined as an independent variable, the regression analysis being performed while correcting the current integrated value based on a value of a coefficient of determination so that a result of the regression analysis has predetermined accuracy; and an estimation unit configured to estimate a capacity of the secondary battery system based on the result of the regression analysis. a current integrated value calculation unit configured to calculate a current integrated value by integrating electric currents of a secondary battery system (23-1,..., 23-M) whose capacity is to be estimated, the electric currents being detected by a current sensor; an SOC estimate-value calculation unit configured to calculate an SOC estimate value, based on a closed circuit voltage, in a stabilization state where a change in SOC of a secondary battery per unit time is comparatively small, the secondary battery constituting the secondary battery system; a regression analysis unit configured to perform a regression analysis in which the current integrated value is defined as a dependent variable and the SOC estimate value is defined as an independent variable, the regression analysis being performed while correcting the current integrated value based on a value of a coefficient of determination so that a result of the regression analysis has predetermined accuracy; and an estimation unit configured to estimate a capacity of the secondary battery system based on the result of the regression analysis.