An abnormality diagnostic apparatus for a vehicle, which can accurately determine whether or not there is an abnormality in a voltage detection unit, while curtailing a cost increase, is provided. Output voltages Va, Vb, Vc outputted from voltage detection units are acquired, and differences |Va−Vb|, |Vb−Vc|, |Vc−Va| between the acquired output values are calculated. If any of the calculation results is greater than a determination threshold value, it is determined that there is an abnormality in any of the voltage detection units.