A method of calculating a State Of Charge of a battery in real time, the method comprising: a battery parameter measurement operation for measuring a current, a voltage, a temperature, a degradation degree of the battery; and a real-time SOC calculation operation for calculating an SOC of the battery in real time based on the measured battery parameter; wherein the real-time SOC calculation operation comprises a real-time SOC arithmetic operation for calculating an SOC of a battery in real time using Extended Kalman Filter below (Extended Kalman Filter) (P: error covariance, H: transform coefficient, K: Kalman gain, Q: standard deviation of true value, W: system noise, x: state variable, z: observation value, V: error with respect to observation true value, k: degree of operation, u: additional input value, f(x,u,o): equation for calculating an SOC, I: unit matrix), wherein the real-time SOC calculation operation further comprises a noise variable setting operation for setting a noise variable W set by an external error in the Extended Kalman Filter, and wherein the noise variable setting operation comprises: an external error setting operation for setting an error occurring when each battery parameter is measured; a temporary noise variable setting operation for setting a temporary noise variable to be used as a noise variable of the Extended Kalman Filter; an error calculation operation for calculating an error between an SOC calculated by reflecting the external error and the temporary noise variable and an SOC calculated without reflecting the external error, and a final noise variable detection operation for detecting a final noise variable optimized for the external error; wherein the noise variable setting operation repeatedly performs the error calculation operation by changing the external error and the temporary noise to various values; wherein the final noise variable detection operation sets a temporary noise variable in which an error calculated in the error calculation operation falls within a predetermined range as a final noise variable; and wherein the noise variable of the voltage model is set to be as small as possible and the weight corresponding to the voltage model is set to be large. a battery parameter measurement operation for measuring a current, a voltage, a temperature, a degradation degree of the battery; and a real-time SOC calculation operation for calculating an SOC of the battery in real time based on the measured battery parameter; wherein the real-time SOC calculation operation comprises a real-time SOC arithmetic operation for calculating an SOC of a battery in real time using Extended Kalman Filter below (Extended Kalman Filter) (P: error covariance, H: transform coefficient, K: Kalman gain, Q: standard deviation of true value, W: system noise, x: state variable, z: observation value, V: error with respect to observation true value, k: degree of operation, u: additional input value, f(x,u,o): equation for calculating an SOC, I: unit matrix), wherein the real-time SOC calculation operation further comprises a noise variable setting operation for setting a noise variable W set by an external error in the Extended Kalman Filter, and wherein the noise variable setting operation comprises: an external error setting operation for setting an error occurring when each battery parameter is measured; a temporary noise variable setting operation for setting a temporary noise variable to be used as a noise variable of the Extended Kalman Filter; an error calculation operation for calculating an error between an SOC calculated by reflecting the external error and the temporary noise variable and an SOC calculated without reflecting the external error, and a final noise variable detection operation for detecting a final noise variable optimized for the external error; an external error setting operation for setting an error occurring when each battery parameter is measured; a temporary noise variable setting operation for setting a temporary noise variable to be used as a noise variable of the Extended Kalman Filter; an error calculation operation for calculating an error between an SOC calculated by reflecting the external error and the temporary noise variable and an SOC calculated without reflecting the external error, and a final noise variable detection operation for detecting a final noise variable optimized for the external error; wherein the noise variable setting operation repeatedly performs the error calculation operation by changing the external error and the temporary noise to various values; wherein the final noise variable detection operation sets a temporary noise variable in which an error calculated in the error calculation operation falls within a predetermined range as a final noise variable; and wherein the noise variable of the voltage model is set to be as small as possible and the weight corresponding to the voltage model is set to be large.