1. A battery state prediction system for predicting a capacity maintenance ratio of a battery under control of a computer, comprising: storage means, wherein said storage means stores: data relating to an observation model for calculating said capacity maintenance ratio on the basis of basic data representing an internal state of said battery, and data relating to a transition model for mapping a temporally preceding internal state and deterioration environment onto a current internal state; and means for determining a current internal state of said battery by finding out a transition path of said internal state having the minimum sum of squares of differences between predictive values calculated from said observation model and said transition model and an actual value, on the basis of a value of said battery measured at each time step, wherein said basic data representing said internal state of said battery comprises a feature quantity extracted from a positive and negative AC impedance curve plot and said observation model is based on a nonlinear regression model.