A state estimation apparatus disclosed in the present specification is a battery management unit estimating the state of an assembled battery including a plurality of energy storage devices. The energy storage devices each have a small-variation region II and a small-variation region IV where an OCV variation amount relative to a residual capacity is smaller, and a variation region I, a variation region III, and a variation region V where the OCV variation amount relative to the residual capacity is greater than that in the small-variation region II and the small-variation region IV. The battery management unit estimates whether or not the assembled battery is usable based on a varying position (a variation in position from a first position to a second position) of the variation region III relative to the actual capacity of the plurality of energy storage devices.