To provide a battery open-circuit voltage estimating method and its device for estimating relatively precisely an open-circuit voltage of a battery in a relatively short period of time after the completion of charging/discharging.

An open-circuit voltage measuring means 23a-1 measures open- circuit voltages of the battery in a prescribed cycle after the completion of charging or discharging of the battery, and the measurements are collected by an open-circuit voltage collecting means 23b. With respect to the collected voltages, a computing means 23a-2 finds voltage values each of which is approximated based on the open-circuit voltage within each period of a plurality of predetermined periods, and to which an approximate exponential expression with an exponent of -0.5 or approximating to -0.5 is asymptotic as an assumed open-circuit voltage for each period. An assumed open-circuit voltage for a period minimizing a difference from an assumed open-circuit voltage for an adjoining period is estimated to be the open-circuit voltage of the battery.