In a voltage monitor that measures voltages applied to the terminals of each of a plurality of capacitors constituting a storage section, the each voltage applied to a connecting point between terminals of the capacitors in charging operation, is divided by resistors and the divided voltage is fed into a selector switch. The resistance values used for dividing the each voltage are determined in advance so that a short-circuit between adjacent input terminals of the selector switch allows the divided voltage to have a voltage shift remarkably larger or smaller than the divided voltage obtained under the normal charging operation.