A fuel cell diagnostic apparatus that diagnoses a fuel cell in which a plurality of power generating cells are stacked together includes a voltage applying device that applies voltage from outside of the fuel cell; a magnetic field measuring device that measures a magnetic field in or around the fuel cell when external voltage is being applied; and a diagnostic device that diagnoses the state of the fuel cell from the measurement results of the magnetic field. By diagnosing an in-plane distribution of current in the power generating cells by measuring the magnetic field, the distribution of water in an electrolyte membrane after power is generated can be diagnosed based on the diagnostic results of that in-plane distribution of the current.