A battery State-of-Charge estimation device comprising: a discharge-and-charge current detection means for detecting a discharge-and-charge current value of a battery; a terminal voltage detection means for detecting a terminal voltage value of the battery, a current-integration-method State-of-Charge estimation means adapted to estimate a current-integration-method State-of-Charge based on a State-of Charge obtained by integrating the discharge-and-charge current value inputted from the discharge-and-charge current detection means and a State-of-Charge obtained at previous time, calculate a variance of the current-integration-method State-of-Charge based on information on a detection accuracy of the discharge-and-charge current detection means; an open-circuit-voltage-method State-of-Charge estimation means adapted to estimate an open-circuit-voltage State-of-Charge based on the discharge-and-charge current value inputted from the discharge-and-charge current detection means and the terminal voltage value inputted from the terminal voltage detection means, using a battery equivalent circuit model, the open-circuit-voltage-method State-of-Charge estimation means adapted to calculate a variance of the open-circuit-voltage-method State-of-Charge based on information of detection accuracy of the discharge-and-charge current detection means and information of detection accuracy of the terminal voltage detection means; an error estimation means adapted to estimate an estimate error of the current-integration-method State-of-Charge based on a difference between the open-circuit-voltage-method State-of-Charge and the current-integration-method State-of-Charge, the variance of the open-circuit-voltage-method State-of-Charge and the variance of the current-integration-method State-of-Charge; and a State-of-Charge calculating means adapted to calculate a State-of-Charge of the battery based on the current-integration-method State-of-Charge and the estimate error estimated by the error estimation means. a discharge-and-charge current detection means for detecting a discharge-and-charge current value of a battery; a terminal voltage detection means for detecting a terminal voltage value of the battery, a current-integration-method State-of-Charge estimation means adapted to estimate a current-integration-method State-of-Charge based on a State-of Charge obtained by integrating the discharge-and-charge current value inputted from the discharge-and-charge current detection means and a State-of-Charge obtained at previous time, calculate a variance of the current-integration-method State-of-Charge based on information on a detection accuracy of the discharge-and-charge current detection means; an open-circuit-voltage-method State-of-Charge estimation means adapted to estimate an open-circuit-voltage State-of-Charge based on the discharge-and-charge current value inputted from the discharge-and-charge current detection means and the terminal voltage value inputted from the terminal voltage detection means, using a battery equivalent circuit model, the open-circuit-voltage-method State-of-Charge estimation means adapted to calculate a variance of the open-circuit-voltage-method State-of-Charge based on information of detection accuracy of the discharge-and-charge current detection means and information of detection accuracy of the terminal voltage detection means; an error estimation means adapted to estimate an estimate error of the current-integration-method State-of-Charge based on a difference between the open-circuit-voltage-method State-of-Charge and the current-integration-method State-of-Charge, the variance of the open-circuit-voltage-method State-of-Charge and the variance of the current-integration-method State-of-Charge; and a State-of-Charge calculating means adapted to calculate a State-of-Charge of the battery based on the current-integration-method State-of-Charge and the estimate error estimated by the error estimation means.