To learn a parameter of a simulation model of a battery efficiently. A battery internal state estimating apparatus, estimating an internal state of a battery based on a simulation model of the battery, includes a storing section (RAM10c) that stores a plurality of parameters of the simulation model, a detecting section (I/F10d) that detects a discharge current flowing from the battery to a load, a selecting section (CPU 10a) that selects a parameter to be subjected to adaptive learning based on a value of the discharge current detected by the detecting section, and an adaptive learning section (CPU 10a) that performs adapting learning on a parameter selected by the selecting section.