A battery state of charging estimating apparatus, comprising: a first SOC calculating unit configured to apply a predetermined parameter to calculate a first state of charging of a battery; a plurality of second SOC calculating units individually configured to apply different parameters, which are values related with one or more of a current, a voltage, and a temperature of the battery, to calculate a plurality of second states of charging (SOCs) of the battery; and an optimal parameter extracting unit configured to: - determine a voltage value at a predetermined time as an open circuit voltage of the battery, when a current which flows in the battery is equal to or below a predetermined current value for a predetermined time or longer; - calculate an actual state of charging of the battery using the open circuit voltage; - identify among the plurality of second states of charging, the second states of charging which is closest to the calculated actual state of charging; - extract, as optimal parameter, the parameter used by the second SOC calculating unit which calculated the closest second state of charging, wherein the first SOC calculating unit applies the optimal parameter to calculate a final state of charging of the battery, wherein the plurality of second SOC calculating units calculates the plurality of second states of charging simultaneously when the first SOC calculating unit calculates the first state of charging. a first SOC calculating unit configured to apply a predetermined parameter to calculate a first state of charging of a battery; a plurality of second SOC calculating units individually configured to apply different parameters, which are values related with one or more of a current, a voltage, and a temperature of the battery, to calculate a plurality of second states of charging (SOCs) of the battery; and an optimal parameter extracting unit configured to: - determine a voltage value at a predetermined time as an open circuit voltage of the battery, when a current which flows in the battery is equal to or below a predetermined current value for a predetermined time or longer; - calculate an actual state of charging of the battery using the open circuit voltage; - identify among the plurality of second states of charging, the second states of charging which is closest to the calculated actual state of charging; - extract, as optimal parameter, the parameter used by the second SOC calculating unit which calculated the closest second state of charging, - determine a voltage value at a predetermined time as an open circuit voltage of the battery, when a current which flows in the battery is equal to or below a predetermined current value for a predetermined time or longer; - calculate an actual state of charging of the battery using the open circuit voltage; - identify among the plurality of second states of charging, the second states of charging which is closest to the calculated actual state of charging; - extract, as optimal parameter, the parameter used by the second SOC calculating unit which calculated the closest second state of charging, wherein the first SOC calculating unit applies the optimal parameter to calculate a final state of charging of the battery, wherein the plurality of second SOC calculating units calculates the plurality of second states of charging simultaneously when the first SOC calculating unit calculates the first state of charging.