A power supply apparatus has a battery that is composed of a plurality of battery modules connected in series, a voltage detector that detects the relative voltages at the individual nodes between the battery modules with respect to a predetermined reference node, an erroneous detection checker that checks whether each of the detected relative voltages is an erroneously detected voltage and a normally detected voltage, and a calculation circuit that calculates the voltages of the individual battery modules based on those relative voltages that are judged to be normally detected voltages. If any of the nodes is recognized as an erroneous detection node as a result of an erroneously detected voltage being detected thereat, the calculation circuit calculates, based on the relative voltages detected at the two nodes located adjacent to the erroneous detection node, a recovery voltage, and then, by replacing the relative voltage detected at the erroneous detection node with the recovery voltage, calculates the voltages of the battery modules.