1. A battery state prediction method for predicting a capacity maintenance ratio of a battery under control of a computer, the method comprising: preparing an observation model for calculating said capacity maintenance ratio on the basis of basic data representing an internal state of said battery, and a transition model for mapping a temporally preceding internal state and deterioration environment onto a current internal state; and determining said current internal state of said battery by finding out a transition path of said internal state having a 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.