An apparatus is provided to calculate a quantity indicating a charged state of an on-vehicle battery. The battery powers a starter starting up an on-vehicle engine. In the apparatus, a plurality of pairs of data consisting of current and voltage of the battery are acquired at predetermined sampling intervals during a cranking period of the engine in response to starting up the starter. At intervals, a value of an internal resistance of the battery is calculated based on the plurality of pairs of data of current and voltage. The internal resistance is one kind of the charged-state indicating quantity. An open voltage difference is calculated, which is a difference between a pseudo circuit-open voltage of the battery given before starting up the starter and a pseudo circuit-open voltage of the battery given after the cranking period. The value of the internal resistance is corrected using the open voltage difference.