A diagnostic apparatus includes a capacitor capable of being connected in parallel with a first battery, first switches that switch the connection state between a plurality of the first batteries and the capacitor, a detection circuit, a second switch that switches the connection state between the capacitor and the detection circuit, a changeover switch that switches the connection state between a second battery and the capacitor, a controller, and a diagnostic unit. The controller turns on the changeover switch to apply a voltage to the capacitor from the second battery, the detection circuit subsequently detects a potential difference or a discharge current, and the diagnostic unit diagnoses at least one of the capacitor, a lowermost first switch, and the second switch.