A remaining-battery-capacity estimating apparatus estimates the remaining capacity of a battery used as a backup. The apparatus reduces the voltage output from a rectifier to an actual load, measures current flowing from the rectifier and the battery to the actual load and a dummy load device provided separately from the actual load, controls the load of the dummy load device such that the measured current is a predetermined value, and calculates the remaining capacity of the battery based on the terminal voltage of the battery and the discharging time of discharging the battery to the actual load and the dummy load device, by referring to battery data indicating the relationship among the discharging time of the battery, the terminal voltage of the battery, and the remaining capacity of the battery, at a predetermined current of the battery.