A vehicle battery's open circuit voltage estimating method and a system therefor for relatively correctly estimating an open circuit voltage of a battery on a vehicle within a relatively short period after completion of the charge or the discharge also without a temperature correction is provided. A disconnected voltage measuring means 23a-1 measures a disconnected voltage of the battery multiple times in a definite period of time after a predetermined period of time after completion of charge or discharge of the battery. An approximate expression determining means 23a-2 determines a power approximate expression having a negative power number by using a difference value of the measured disconnected voltage and an assumed open circuit voltage. And, an arithmetic operation controlling means 23a-3 makes the approximate expression determining means update the assumed open circuit voltage and determine the power approximate expression repeatedly until the power number of the determined power approximate expression becomes −0.5 or about −0.5, and the assumed open circuit voltage is estimated to be the open circuit voltage when the power number becomes −0.5 or about −0.5.