An apparatus for accurately estimating the SOC of a rechargeable battery installed in a hybrid electric vehicle. A voltage variation measurement unit measures terminal voltage variation amount of the rechargeable battery in a no-battery-current state in which a charge-discharge current does not flow in the rechargeable battery. A polarization voltage calculator calculates a polarization voltage from the measured variation amount. An electromotive force calculator subtracts the polarization voltage from a non-load voltage in a state in which a charge-discharge path is open to calculate the electromotive force of the rechargeable battery. An SOC estimator estimates the SOC of the rechargeable battery based on the electromotive force.