An abnormality diagnosis apparatus performs: a first step of diagnosing a shortage of precharge voltage or a short-circuit failure between electrodes of a capacitor based on a detection value of a precharge voltage detection circuit in a state where a first relay and a second relay are turned off, and a precharge circuit is operated;, a second step of diagnosing, after a normal determination in the first step, a disconnection failure of a precharge path based on the detection value of the precharge voltage detection circuit or a detection value of an inter-relay voltage detection circuit in a state where the first relay and the second relay are turned off, and the precharge circuit is not operated; and a relay failure diagnosis step of diagnosing, after a normal determination in the second step, a short-circuit failure and an open failure of the first relay or the second relay.