In a proposed apparatus and method, constant-voltage charge is performed with an in-vehicle secondary battery immediately after start of a vehicle or during running of the vehicle. A quantity relevant to polarization caused in the battery immediately after start of the constant-voltage charge is calculated using data of the charge current. It is determined whether or not a change rate of the calculated polarization-relevant quantity is less than a given threshold. When the change rate is less than the given threshold, a plurality of data of the charge current sampled and held during a predetermined period of time are acquired. A value of the charge current to be accumulated until the charge current reaches a given final value is calculated using the plurality of data of the charge current. The internal electric state of the battery is estimated based on the accumulated value of the charge current.