There is provided an apparatus that calculates a polarization voltage of a secondary battery. A temperature sensor detects a temperature of the secondary battery; a voltage sensor detects a voltage of the secondary battery; and a current sensor detects an electric current of the secondary battery. A battery ECU calculates a polarization voltage based on the electric current and adaptively sets an upper limit value and a lower limit value of the polarization voltage according to a temperature characteristic of the secondary battery. The calculated polarization voltage is compared with an upper limit value and a lower limit value, whereby the polarization voltage is corrected. An SOC is estimated based on the corrected polarization voltage.