Provided is a full-charge-capacity estimating device that has one or more of a plurality of battery modules, as battery-modules-to-be-measured, charged or discharged by means of a first switch element and a second switch element according to whether a power supply device is in a powering state or a regenerating state, measures an integrated current value and a change in the state-of-charge of the battery-module-to-be-measured, and then estimates the full charge capacity of the battery-module-to-be-measured from the integrated current value and the change in the state-of-charge.