1. A neural network type of apparatus for detecting an internal state of a secondary battery implemented in a battery system, the apparatus comprising: detecting means for detecting electric signals indicating an operating state of the battery; producing means for producing, using the electric signals, an input parameter required for estimating the internal state of the battery, the input parameter reflecting calibration of a present charged state of the battery; and estimating means for estimating an output parameter indicating the charged state of the battery by applying the input parameter to neural network calculation.