A method of calculating an available capacity includes the steps of: calculating a first estimated SOC from a value in voltage (S 100); calculating an integral of a value of a current (S 100) and adding the integral to calculate a second estimated SOC (S 120); calculating a difference between the first and second estimated SOCs (S 130); calculating a first correction coefficient R(1) from the value in voltage (S 140); calculating a second correction coefficient R(2) to be smaller for the current having a smaller value (S 150); and adding the second estimated SOC to an correction term, or the dSOC multiplied by R(1) and R(2), to calculate a corrected SOC (S160).